An exceptionally short and simple enantioselective total synthesis of pentacyclic triterpenes of the beta-amyrin family

A new and very direct enantioselective total synthesis of members of the be ta-amyrin family of pentacyclic triterpenes has been developed starting wit h acylsilane 5, 2-propenyllithium, and cyclohexenylmethyl bromide 6, which were assembled to form tetraene 7, Cationic cyclization of 7 and silylation afforded 8, which after vinyl triflate formation was cyclized via a Cu(I) intermediate (Scheme 2) to form the TBS ether of aegiceradienol 10, a versa tile intermediate that is readily converted into natural beta-amyrins such as beta-amyrin (1) and oleanolic acid (2). The C(14)-diastereomer (13) of a egiceradienol was also synthesized from the C(14)diastereomer of 8 using an intramolecular Stille reaction for the closure of ring D (Scheme 4).