Cleavage of Cyclic Ethers with Boron Bromide. A Convenient Route to the Bromosubstituted Alcohols, Aldehydes and Ketones

Abstract

Cyclic ethers are r e a d i l y cleaved by BBr3 under m i l d conditions, providing the corresponding 5 -b r m a l k y l borates (1).Redistribution of 1 w i t h methanol affords w-branoalcohols (2).Oxidation of 1 w i t h pyridinium chlorochranate forms the corresponding g-bromoaldehydes (3).Under these conditions, epoxides y i e l d f i r s t the borates o f the corresponding bromohydrins, w i t h subsequent oxidation of the intermediate (without i s o l a t i o n ) , g i v i n g the a-bromoketones i n high p u r i t y and satisfactory y i e l d .Heterobifunctional alkanes play an important r o l e i n organic synthesis.I n the preparation o f unsaturated straight-chain canpounds via the W i t t i g reaction, w-haloaldehydes o r alcohols serve as useful intermediates.For example, w-branoalcohols are valuable s t a r t i n g materials i n the synthesis o f sane insect and ~r o s t a g l a n d i n s .~ However, a systematic study of the preparation o f such canpounds frm simple precursors has not beewreported i n the l i t e r a t u r e .W e wish t o report a convenient and general approach f o r the preparation o f such derivatives from the r e a d i l y available c y c l i c ethers.The use o f boron halides as ether cleavage reagents i s well-documented i n the l i t e r a t u r e .4Lately, boron b r m i d e (BBr3) has become the reagent o f choice f o r such reaction^.^It i s more effective than boron chloride (BC13) and reacts cleanly and q u a n t i t a t i v e l y under m i l d conditions.Consequently, we undertook t o examine the cleavage o f c y c l i c ethers w i t h BBr3 as a potential synt h e t i c route t o g-bmmoalcohols, pbranoaldehydes, and a-bromoketones.Representative c y c l i c ethers react smoothly w i t h B B r 3 The ether was added t o a solution of BBrj i n C H p 2 a t O' C i n order t o avoid any possible polymerization of the c y c l i c ether.After the i n i t i a l exothermic reaction subsided, the mixture was heated f o r 1 h under r e f l u x (40°C).'forming 11 the g-brmoalkylborate U) q u a n t i t a t i v e l y ( B nmr: S 18 ppm, t y p i c a l of borate'esters) ( ~4 ' 1 ) .t Dedicated t o Professor Herbert C.