H

42: R = Ph, Ar = 2-pyr¡dyl, (no additive) 43: R = Ph, 76%

Scheme 10

be trapped. The trapping with a sugar acetylene to give the a-C-glycoside 44 is illustrated in Scheme 11.

Hung and Wong used glycosyl phosphates as radical precursors en route to C-glycosides via intramolecular samarium diiodide mediated cyclization as shown in Scheme 11 [16]. Work by Czernecki et al. focused on intramolecular additions of 2-deoxy-2-amino sugars. Exposure of the diallylated species 47 to tin hydride in the presence of triethylborane gave compound 48 in 74% yield as the sole product [17]. Lee et al. carried out intramolecular cyclizations to C-glycoside-like structures using aldehydes such as 49. Exposure of 49 to tributyltin hydride and AIBN gave an O-stannyl ketyl intermediate that underwent 5-exo cyclization to give 50 and 51 in a 46:54 ratio in 91% combined yield, (Scheme 11) [18].

BnO OBn O 54

Scheme 12

Although not radical in nature, this class of species is electron deficient and their inclusion here is warranted. Vasella has continued his work on glycosyl car-benes. The carbene generated from 52, when trapped with dimethyl fumarate of N-phenylmaleimide, gave 53 and 54, respectively (Scheme 12) [19].

0 0

Post a comment