Cetalox (R) and analogues: Synthesis via acid-mediated polyene cyclizations

Using a novel, acid-mediated cyclization methodology, a direct access to Cetalox (R) ((+/-)-1; a commercially important ambergris-type odorant) and various structurally related didehydro (i.e., 19, 26, and 30) and tetradehydro (i.e., 28 and 37/38) analogues is described. Treatment of either (E,E)-14 or (E)-15 with an excess of FSO3H in 2-nitropropane at -90 degrees stereospecifically afforded (+/-)-1 in 40 and 42% yield, respectively. Under similar conditions, cyclization of (E)-18 or 20 furnished 19 in 60 and 64% yield, respectively. Analogously, using an excess Of ClSO3H in CH2Cl2 at -80 degrees, 26 is formed with high stereoselectivity by cyclization of either (E)-24 or (Z)-25 (52 and 31% yield, resp.); in the same manner, 28 was prepared from 27 (22% yield). The same principle was applied to the synthesis of racemic Superambrox (30), via cyclization of 35, but only with poor selectivity (22%) and low yield (7%). Another approach via cyclization of (E)-40 under solvolysis conditions (excess TFA in CH2Cl2 at - 10 degrees) gave a higher yield (15%) with improved selectivity (43%). Finally, cyclization of 34 (1:1 diastereoisomer mixture) afforded 37/38 (10: 1) in 27% yield. The qualitative organoleptic properties of 19, 26, 28, 30, and 37/38 (10:1) are briefly discussed.