Steric Effects on Reaction Rates. VI. Application of a New MM2 Force‐Field for Carbenium Ions to Solvolysis Rates of Secondary p‐Toluenesulfonates

Abstract

An empirical force-field for carbenium ions has been incorporated in Allinger's MM2 programme. Structural parameters of secondary carbenium ions calculated by this method are compared with those obtained with Schleyer's BIGSTRN calculations. The strain changes occurring upon solvolysis of secondary p-toluenesulfonates are evaluated by means of this force-field and correlated with the rate constants for solvolysis. The equation for correlation of acetolysis, relative to cyclohexyl p-toluenesulfonate, of 28 k, substrates is AGZ, = 0.67 AE,, -0.20 (r = 0.958).

Synthesis and evaluation of cyclobutylcarbinyl derivatives as potential intermediates in diterpene biosynthesis

“…The reductive removal of the C-19 hydroxyl group was carried out by the procedures of Crossley and Dowell. 24 A solution of 1.10 g (2.8 mmol) of ketal alcohol 6 and 3.5 g (35 mmol) of triethylamine in 50 mL of pentane was stirred at room temperature as 500 mg (4.4 mmol) of methanesulfonyl chloride was added. The solution was allowed to stir for about 2 h until the TLC spot for the starting ketal alcohol had disappeared.…”

“…Stirring was continued for 24 h, and the progress of the reaction was monitored by GC and TLC. Six additional 0.65-mL (8 mmol) portions of ethyl formate were added during the course of reaction.…”

“…23 The hydroxyl group at the hindered C-19 position was conveniently removed by lithium ammonia reduction of the corresponding phenylthio ether according to the procedures of Crossley and Dowell. 24 The known beyeran-16one (7)26 was liberated by hydrolysis of the ketal. Condensation of 7 with ethyl formate in the presence of sodium (18) Wiberg, K. B.; Hess, B.…”

“…(23) This reaction sequence for converting isosteviol methyl ester to beyeran-16-one was developed earlier in our laboratory by David A. Ley: Ph.D. Thesis, University of Illinois, Urbana, 1976. (24) Crossley, N. S.; Dowell, R. J. Chem. hydride, or sodium and potassium hydrides together,26 in tetrahydrofuran (THF) at room temperature was sluggish and at best 44-49% yields of the a-hydroxymethylene ketone 8a were realized.…”