A REPETITIVE APPROACH TO THE SYNTHESIS OF MEDIUM AND LARGE RING COMPOUNDS WITH A RING-OPENING RING-CLOSURE CASCADE REACTION OF SILOXYCYCLOPROPANE DERIVATIVES AS CRUCIAL STEP/

Starting from siloxycyclopropyl-substituted dimethyl malonates 1 and 2 a chain elongation was performed to give new malonates 8, 9, 10, and 18 in reasonable overall yield. Further elongation by five carbon atom s transforms 10 into 15. Precursor 21 was prepared by alkylation of cy clopropanecarboxylate 20 with dibromide 17 and subsequent treatment wi th sodium dimethyl malonate. Compounds 8, 9, 10, 15, 18, and 21 were s ubjected to cesium fluoride under high dilution which induced a ring-o pening/ring-closure cascade reaction giving cyclopentadecanone derivat ives 11, 12, 13, cycloeicosanone derivative 16, cyclotetradecanone der ivative 19 and cyclononane derivative 22 (together with 23). The effic iency of this reaction including an intramolecular Michael addition is discussed. The sequence allows efficient synthesis of medium and larg e carbocycles by a highly flexible building block system involving in principle unlimited repetition of construction steps. An X-ray analysi s of 19 reveals its crown-like conformation which is slightly distorte d to a more planar skeleton compared with a ten-membered ring analog 2 4.