The excited state meta effect, also known as the meta-ortho effect, results from selective electron transmission from an electron-donating group to the meta and ortho sites on an aromatic ring in its first excited singlet state. This effect facilitates photochemical cleavage of benzylic C–O or C–N bond to release the corresponding alcohol, carboxylic acid, or amine when an electron-donating amino group is at the meta position, as demonstrated in our recent work of using a 3-diethylaminobenzyl (DEABn) group as an effective photolabile protecting group (PPG). Herein, we demonstrate that an ortho amino group can also facilitate benzylic C–O bond cleavage to release an alcohol or carboxylic acid. However, an amino group at the meta position results in a PPG with better overall chemical and photochemical properties.

Two structurally simple photolabile protecting groups for releasing 1,2- and 1,3-diols have been developed. The diols can be protected in high yields and released from their corresponding acetals with high chemical efficiency.

Three QS-21-based vaccine adjuvant candidates with a terminal-functionalized side chain incorporated in the west wing trisaccharide have been synthesized. The terminal polar functional group serves to increase the solubility of these analogues in water. Two of the synthetic analogues have been shown to have adjuvant activity comparable to that of GPI-0100. The stand-alone adjuvant activity of the new synthetic analogues again confirmed that it is a feasible way to develop new saponin-based vaccine adjuvants through derivatizing at the west wing branched trisaccharide domain. Inclusion of an additional polar functional group such as a carboxyl group (as in 3x) or a monosaccharide (as in 4x and 5x) is sufficient to increase the water solubility of the corresponding synthetic analogues to a level comparable to that of GPI-0100 and suitable for immunological studies and clinical application. The structure of the incorporated side chain has a significant impact on the adjuvant activity in terms of the magnitude and nature of the host’s responses.

The 3-(diethylamino)benzyl (DEABn) group has been studied for releasing primary, secondary, and tertiary amines by direct photochemical breaking of the benzylic C–N bond. While photochemical release of primary and secondary amines provides high yields in methanol, release of tertiary amines in MeCN/water can improve yields and reduce the undesired dealkylation side reaction.

Structurally simple benzyl-type photolabile protecting groups (PPGs) have been developed to release alcohols and carboxylic acids. Release of two substrates from one PPG chromophore has also been accomplished.

A new photochemical method of C–N bond formation has been developed. A properly substituted trityl alcohol can cleave the benzylic C–O bond and replace it with a C–N bond which is stable under the irradiation conditions. The C–N bond can then be photochemically cleaved with the same light source when the nitrogen is protonated.

New photoresponsive block copolymers (BCPs) have been developed by incorporating photocleavable units. These photocleavable units are derived from robust photolabile protecting groups (PPGs); UV irradiation cleaves the BCPs and releases a well-defined chemically active functional group at the cleaved end of a PS polymer.

Studies have been conducted to elucidate the mechanism of glycosylation reactions using a prop-1-enyl donor isomerized directly from an allyl glycoside. The reactions promoted by NIS/TfOH can take place in high yields in acetonitrile at room temperature. Activation of the anomeric prop-1-enyl group often leads to both the desired glycoside (e.g., 9) and the addition-reaction product (e.g., the anomeric mixed acetal 10). TfOH perhaps has a dual role in the reaction: i.e., (a) producing IOTf in situ to activate the prop-1-enyl group and (b) catalyzing the transformation from the addition-reaction product to the desired glycoside (e.g., from 10 to 9). The latter process involves multiple competing pathways.

Three structurally defined QS-21-based immune adjuvant candidates (2a–2c) have been synthesized. Application of the two-stage activation glycosylation approach utilizing allyl glycoside building blocks improved the synthetic accessibility of the new adjuvants. The efficient synthesis and establishment of the stand-alone adjuvanticity of the examined synthetic adjuvant (2b) open the door to the pursuit of a new series of structurally defined QS-saponin-based synthetic adjuvants.

Hydrophilic photolabile protecting groups (PPGs) for hydroxyl protection have been developed. The new PPGs are derived from 3-(dimethylamino)trityl (DMATr) by replacing the two methyl groups with two hydrophilic butyryl groups. The new PPG reagents can be readily prepared and installed. They are stable in the dark but can be removed cleanly and efficiently in aqueous environments upon irradiation with a UV lamp or sunlight.

A highly efficient glycosylation protocol recently developed in our laboratory has been utilized in the short synthesis of the tetrasaccharide 1, an antigen important to the development of carbohydrate-based diagnostic tools and vaccines against anthrax. The protocol employs allyl glycosides as building blocks and improves the overall synthetic efficiency.

A series of trityl-based photolabile hydroxyl protecting groups have been examined. These PPGs evolve from the traditional acid-labile trityl protecting group with proper electron-donating substituents. Structure–reactivity relationships have been explored. A m-dimethylamino group is crucial to achieve high photochemical deprotection efficiency. The o-hydroxyl group in 8 greatly improves the yield of the photochemical deprotection reaction, compared with the corresponding o-methoxyl-substituted counterpart 7. However, comparison between the photoreactions of 9 and 11 does not show similar structural relevance. The PPG in ether 1 (i.e., DMATr group) is structurally simple and easy to prepare and install. Its deprotection can be successfully carried out with irradiation of sunlight without requirement of photochemical devices.

A novel oxidation approach utilizing a robust photolabile carbonyl protecting group reagent (1) as the oxidizing reagent has been developed. Different from existing methods, this approach oxidizes primary alcohols to the photosensitive acetals (e.g., 3), providing another unique approach to the protected aldehydes. Thus, for the first time, oxidation and protection are achieved in one reaction. Secondary alcohols are oxidized to the corresponding ketones. Moreover, the photolabile protecting group (PPG) also oxidizes ethers and esters. The oxidation is presumably via hydride abstraction by the tritylium ion generated from 1 under acidic conditions. However, the mechanisms for primary alcohols and secondary alcohols are slightly different.

New salicyl alcohol derived photolabile carbonyl protecting groups have been developed, and the effect of substituents on the photochemical properties of photolabile protecting groups (PPGs) has been studied. The 3-(dimethylamino)phenyl groups at the α position prove to be important to the efficiency of the deprotection reactions, as shown in the photo reactions of the acetal 9. On the other hand, expansion of the salicyl alcohol’s benzene skeleton to naphthalene does not improve the photochemical properties of PPGs. A neutral protecting protocol has been generalized to new PPGs with α,α-diaryl salicyl alcohol backbone. Thus, installation of PPGs onto aldehydes is readily achieved at 140 °C without using any other chemical reagents. These PPGs are stable under acidic conditions typical for hydrolyzing acetals and constitute orthogonal protecting groups with traditional 1,3-dioxane/1,3-dioxolane for carbonyl compounds. Highly efficient release of carbohydrate molecules is demonstrated, which can be potentially useful in site-specific release and immobilization of carbohydrates for preparation of high-density microarrays. With the enriched PPG toolbox, PPGs are divided into three subgroups based on their UV absorption profiles. PPGs from different subgroups can be sequentially removed by using different UV irradiation wavelengths. For PPGs absorbing UVA (λ >315 nm), photochemical deprotection can be carried out with sunlight in high yields.

A new photolabile hydroxyl-protecting group has been developed by introducing a dimethylamino group to the meta position of an aromatic ring of the traditional trityl (Tr) protecting group.

A practical, useful glycosylation method employing only allyl glycoside building blocks has been developed. The donor's glycosylation reactivity is turned on via isomerization of its anomeric allyl protecting group into the corresponding prop-1-enyl moiety. Subsequent chemoselective activation with NIS/TfOH achieves high yields in glycosidic bond construction at room temperature. The efficacy and efficiency of this approach in carbohydrate synthesis is demonstrated in the concise synthesis of the fully protected Shigella flexneri serotype Y O-antigen.

A concise route to ethyl 7-bromo-1-cyclopropyl-6,8-difluoro-4-quinolone-3-carboxylate has been developed. This compound is a key intermediate for divergent synthesis of various C7-substituted fluoroquinolones, a group of potent topoisomerase II inhibitors with promising clinical applications.

•Structurally simple and practically useful PPGs are developed.•The PPGs release various functional groups efficiently.•Photo deprotection can occur in organic, aqueous, and solid phase.•The PPGs are easy to prepare and install, with remarkable dark stability.We have recently developed a number of photolabile protecting groups (PPGs) for efficient release of carbonyl, hydroxyl, diol, carboxyl, and amino functionalities. These new PPGs are designed on the basis of the excited state meta effect. They share common features including structural simplicity, easy preparation, flexible structural modification, diverse protection methods, high protection and deprotection efficiency, tolerance of ambient conditions during irradiation, and remarkable stability toward chemical treatments. They can be potentially useful in a wide range of applications including organic synthesis, caging, and materials science.Download high-res image (114KB)Download full-size image

A new photolabile hydroxyl-protecting group has been developed by introducing a dimethylamino group to the meta position of an aromatic ring of the traditional trityl (Tr) protecting group.

New photoresponsive block copolymers (BCPs) have been developed by incorporating photocleavable units. These photocleavable units are derived from robust photolabile protecting groups (PPGs); UV irradiation cleaves the BCPs and releases a well-defined chemically active functional group at the cleaved end of a PS polymer.

A practical, useful glycosylation method employing only allyl glycoside building blocks has been developed. The donor's glycosylation reactivity is turned on via isomerization of its anomeric allyl protecting group into the corresponding prop-1-enyl moiety. Subsequent chemoselective activation with NIS/TfOH achieves high yields in glycosidic bond construction at room temperature. The efficacy and efficiency of this approach in carbohydrate synthesis is demonstrated in the concise synthesis of the fully protected Shigella flexneri serotype Y O-antigen.