Highly Diastereoselective Aziridination of Imines with Trimethylsilyldiazomethane. Subsequent Silyl Substitution with Electrophiles, Ring Opening, and Metalation of C-SilylaziridinesA Cornucopia of Highly Selective Transformations

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Abstract

Treatment of a range of N-sulfonyl (Ts and SES) imines derived from aromatic, heteroaromatic, aliphatic, and unsaturated aldehydes with trimethylsilydiazomethane gave C-silylaziridines in good yield (32-83%) and with high diastereoselectivity in favor of the cis product (80:20-100:0). In contrast, an alpha-imino ester gave predominantly the trans-aziridine (89:11) in high yield (91%). The synthetic potential of C-silylaziridines was investigated. Treatment with F(-) (tetrabutylammonium triphenyldifluorosilicate was used) in the presence of aldehydes gave the alpha-hydroxyaziridines in high yield and high diastereoselectivity (86:14-98:2) for the newly created stereogenic center. Complete retention of configuration was observed in the substitution of the silyl group with electrophiles in all cases. Trapping with deuterium (using CDCl(3) as electrophile) was also successful, but trapping with phosphate [using ClP(O)(OPh)(2)] and acetate (using Ac(2)O) was unsuccessful. In these latter cases ring opening by chloride and acetate, respectively, was observed. Further ring-opening reactions were effected using azide and thiolate nucleophiles and in all cases complete regioselectivity in favor of attack at the silyl-bearing carbon occurred. Complete regioselectivity was also observed in the carbonylative ring expansion using Co(2)(CO)(8) to give a beta-lactam. Treatment of cis-1-tosyl-2-phenyl/butyl-3-trimethylsilylaziridines with n-BuLi and subsequent quenching with MeI followed completely different pathways, depending on the 2-substituent. In the case of the 2-phenylaziridine, metalation was initiated alpha to the phenyl group and led finally to a fused tricyclic adduct with four stereogenic centers as a single diastereoisomer. In the case of the 2-butylaziridine, metalation occurred alpha to the silyl group and led to a trisubstituted silylaziridine, probably via an azirine intermediate.

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