Glycosides, -Wittig rearrangements

Where the Wittig rearrangement provides products formed with retention of the configuration at the anomeric center, the use of carbenoids in the formation of C-glycosides provides a complementary... 

This section focuses on the use of concerted processes in the preparation of C-glycosides. The topics covered are cycloaddition reactions and Wittig rearrangements as approaches to C-glycoside synthesis. 

The [1,2]-Wittig rearrangement has found several applications in the stereocontrolled conversion of 0-glycosides to C-glycosides. In a representative case, treatment of 73 with -BuLi provided 74 in 77% yield with > 98 2 The products of these reactions are potentially of biological... 

Another protocol represents the first example of a C-glycoside synthesis based on a [1,4]-Wittig rearrangement (eq 37). Moreover, interception of the intermediate lithium enolate with an external electrophile is achieved. In addition, the application of this synthetic strategy for the synthesis of zaragozic acid A is demonstrated. ... 

The protected methyl glycoside 3 is converted to the corresponding aldehyde by Swern oxidation using oxalyl chloride activated DMSO. Further reaction with triethyl phosphonoacetate and sodium hydride -known as the Horner-Wadsworth-Emmons reaction - provides selectively the trans et /Tun saturated ester 4 in 72 % yield. This valuable alternative to the Wittig olefination protocol uses phosphonate esters as substrates which are readily available from alkyl halides and trialkyl phosphites via the Arbuzov rearrangement.9 co2Et Reaction of the phosphonate with a suitable base gives the... 

Chirality transfer through a Still-Wittig reaction provided the C-glycoside (113) from the enol ether (equation 29). The low yield (25%) was mainly due to protonation of the intervening oxyanion to the methyl ether (114) prior to rearrangement. Presumably, this side reaction could have been suppressed with HMPA as cosolvent. ... 

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