Application of Electrocyclic Ring-Opening and Desymmetrizing Nucleophilic Trappings of meso-6,6-Dibromobicyclo[3.1.0]hexanes to Total Syntheses of Crinine and Haemanthamine Alkaloids

Abstract

The thermally induced electrocyclic ring-opening of C2-symmetric (meso) 6,6-dibromobicyclo[3.1.0]hexanes such as 10 in the presence of the chiral, nonracemic 1°-amine 28 afforded a ca. 1:1 mixture of the diastereoisomeric and chromatographically separable 1-amino-2-bromo-2-cyclohexenes 37 (42%) and 38 (45%). Each of these was elaborated over 13 steps, including Suzuki–Miyaura cross-coupling, radical cyclization, and Pictet–Spengler reactions, into (−)- or (+)-crinane (1 or ent-1, respectively). Variations on these protocols were applied to the total syntheses of (+)- and (−)-11-hydroxyvattitine [(+)- and (−)-3], (+)- and (−)-bulbispermine [(+)- and (−)-4], (+)- and (−)-haemanthamine [(+)- and (−)-5], (+)- and (−)-pretazettine [(+)- and (−)-6], and (+)- and (−)-tazettine [(+)- and (−)-7] as well as (±)-hamayne [(±)-8] and (±)-apohaemanthamine [(±)-9]. A number of these alkaloids were synthesized for the first time.