Studies toward the Synthesis of Azadirachtin,Part 2:Construction of Fully Functionalized ABCD Ring Frameworks and Unusual Intramolecular Reactions Induced by Close-Proximity Effects

摘要

In the preceding Communication in this issue'1' we described the total synthesis of a potential decalin precursor to azadirachtin (I),'2' its coupling to a suitable norbornene fragment 4 (Scheme 1),and the elaboration of the product to an advanced intermediate along the path to this synthetic target.Herein we report the total synthesis and semisynthesis from azadirachtin of a more advanced decalin system 3,its coupling to the same norbornene fragment 4,and the elaboration of the resulting product to an advanced intermediate for the total synthesis of azadirachtin (Scheme 1).This report also includes a number of unusual reactions induced by proximity effects and special steric factors highlighting the unique characteristics of the azadirachtin scaffold.Having successfully synthesized tricyclic decalin system 5,(Scheme 1) and explored its chemistry toward azadirachtin (1) as described in the preceding Communication,'1' we turned our attention to the more advanced tetracyclic decalin precursor 3,which bears the tetrahydrofuran ring system of 1 within its structure.