Anomeric Acetates and Imidates

The formate group has been used at the anomeric center to install acetylenic moieties (445 ^ 446, Scheme 78) [130], while an acetate function served admirably as the activating function on a tetrahydropyran ring to install an allyl group in the synthesis of the F-ring 448 of the spongistatins. The TBS groups were first replaced with acetates to ensure ^-C-glycosidation [131].

C-Glycosidation of furanosyl acetates is still a popular method for introduction of carbon-based groups at the anomeric center. Scheme 79 shows some recent examples. The entries highlight the use of a diethylthiocarbamoyl function as a partic-

Scheme 77

ipating group to control the facial selectivity of C-glycosidation [132]. This is significant, since the lack of a group at C2 hinders stereocontrol in the 2-deoxyribofuranosides [133]. It has also been found that scandium perchlorate serves as a good catalyst for the selective a-C-glycosidation of perbenzylated ribo-furanosyl acetates [134].

Heteroatom-based siloxy dienes have received attention in recent years as an efficient method for building up ring frameworks. The subject has been reviewed, and only a few applications are illustrated here [135]. This is a powerful process, since in a relatively small number of steps several rings can be joined together. The first two equations [136,137] of Scheme 80 show how a single butenolide unit can be attached, while the last sequence illustrates an iterative methodology [138]. It should be noted that in the addition of trimethylsiloxy furan to the anomeric acetate,

Reverse Testicular Atrophy

OBn 453

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