Studies Directed Towards the Total Synthesis of Anticapsin and Related Compounds. II. Diels-Alder Addition of N-Acyl Dehydroalanine Esters to 1-Trimethylsilyloxycyclohexa-1,3-diene

In a study directed towards the total synthesis of anticapsin and related compounds, the Diels-Alder addition of N- acyl dehydroalanine esters (15)-(19) to 1-trimethylsilyloxycyclohexa-1,3-diene (6) was investigated. The cycloadditions were highly regioselective and provided 2-amino-1-trimethylsilyloxybicyclo[2.2.2]oct-5-ene-2-carboxylate derivatives (23)-(28) in moderate to good yields. A modest stereoselectivity was observed in the reactions, with the endo -adducts (23a)-(28a) favoured over the exo-adducts (23b)-(28b) by a ratio of 1.5-3.3:1. Adducts (23a) and (24a) underwent stereoselective epoxidation to the less hindered face of the olefinic double bond. Reaction of the resultant epoxides (31) and (32) with TiCl4 afforded carbolactones (34) and (35), thereby allowing the assignment of endo stereochemistry to adducts (23a) and (24a). The stereochemistry of the remaining adducts was assigned by the use of 1H n.m.r. spectroscopy.