Intramolecular Cyclization of 2,7- or 2,8-Bis-unsaturated Esters Mediated by (η2-Propene)Ti(O-i-Pr)2. Facile Construction of Mono- and Bicyclic Skeletons with Stereoselective Introduction of a Side Chain. A Synthesis of d-Sabinene
1997-10-22T00:00:00Z (GMT) by
tert-Butyl 2-en-7-ynoate 6 was treated with (η2-propene)Ti(O-i-Pr)2 (3), generated in situ from Ti(O-i-Pr)4 or Ti(O-i-Pr)3Cl and i-PrMgCl, in ether at −50 to −20 °C to afford the product 8 in good yield. The presence of the intermediate titanabicycle 7 was verified by bis-deuterolysis with excess D2O. When the titanabicycle 7 was treated with 1.1 equiv of i-PrOD and then worked up as usual, the monodeuterated product 10 was obtained with high site selectivity and stereoselectivity. Other electrophiles such as aldehydes and ketones also reacted with the titanabicycle in a highly stereoselective manner to give cyclopentanes having a stereo-defined side chain. On the contrary, treatment of the corresponding ethyl ester, ethyl 8-(trimethylsilyl)-(E)-2-octen-7-ynoate (28), with 3 under the same conditions followed by the addition of 1.1 equiv of s-BuOH afforded 2-(trimethylsilyl)-1-bicyclo[3.3.0]octen-3-one (32) in 80% yield. Quenching the same reaction mixture with i-PrOD, EtCHO, and Et2CO in place of s-BuOH gave 4-deuterio (with exclusive deuterium incorporation), 4-(1-hydroxypropyl), and 4-(1-ethyl-1-hydroxypropyl) derivatives of the above bicyclic ketone (34, 35, and 36) in good yields. These electrophiles were always introduced from the convex face of the bicyclic skeleton. The stereochemistry of the cyclization could be controlled by an allylic substituent such as (tert-butyl)dimethylsiloxy or butyl group to a high degree yet with a reversal diastereoselection to give 45 or 47. The reaction of ethyl 7-octen-2-ynoate (56) and 3 at −50 to 0 °C took place in a quite different way to afford 1-[(ethoxycarbonyl)methyl]bicyclo[3.1.0]hexane (64) in 74% yield after hydrolysis. If the simple hydrolysis is replaced by deuterolysis or the action of diethyl ketone, 1-[(ethoxycarbonyl)dideuteriomethyl] (with 99% deuterium incorporation), or 1-[(ethoxycarbonyl)(3-pentylidene)methyl] derivative of the above product (65 or 66) was obtained in good yields. A 7-en-2-ynoate having an internal Z-double bond such as 80 afforded a single stereoisomer 82 with the substituent at the endo position of the bicyclic skeleton, suggesting that the stereochemical integrity of the Z-double bond of the starting material was retained in the product. An alkyl substituent at the allylic position of the substrates like 74 and 76 nicely controlled the stereochemistry of the cyclization to afford single products 75 and 77 with the substituent being placed in the exo orientation of the bicyclic structure. This high diastereoselectivity was successfully applied to an enantioselective synthesis of d-sabinene from an optically active enynoate via nearly complete chirality transfer.