Chemo-, regio-, and stereoselectivity in acid-catalyzed hydromethoxylation of tricyclo[4.1.0.02,7]hept-1-yl and 7-methyltricyclo[4.1.0.02,7]hept-1-yl phenyl sulfones

Abstract

Hydromethoxylation of tricyclo[4.1.0.0 2,7 ]hept-1-yl and 7-methyltricyclo[4.1.0.0 2,7 ]hept-1-yl phenyl sulfones with methanol at 20°C in the presence of a catalytic amount of perchloric acid is initiated by the endo attack of proton at the C 1 atom, and the subsequent cleavage of the side C 1 -C 2 bond leads to formation of mixtures of diastereoisomeric exo-7-phenylsulfonyl-2-methoxybicyclo[4.1.0]heptanes, the endo-2 isomer prevailing. Probable factors responsible for the observed chemo-, regio-, and stereoselectivity of the addition are discussed. I, II, X, R = H; III-V, R = Me; VI-VIII, R = Ph.It was reliably established [1, 2] that acid-catalyzed solvation of bicyclo[1.1.0]butane compounds is initiated by proton attack exclusively at the bridgehead position; next follows solvent addition at either central or side C-C bond of the substrate. Acid-catalyzed hydromethoxylation of bicyclo[1.1.0]butane derivatives was studied most thoroughly. The chemo-and regioselectivity of the addition strongly depend on the position and electronic properties of substituents at the bicyclobutane skeleton. In particular, tricyclo[4.1.0.0 2,7 ]-heptane (I), which may be regarded as specific 2,4-dialkyl-substituted bicyclobutane, takes up methanol molecule with strict chemoslelectivity at the side C-C bond [3] with formation of norcarane structure II, whereas the addition to bicyclobutane itself was not chemoselective: both cyclobutyl and cyclopropylcarbinyl derivatives were formed [3,4]. Specific stereochemistry was revealed in acid-catalyzed addition of CH 3 OD to tricycloheptane I [3]. Electrophilic attack by D + on the side bicyclobutane C-C bond is characterized by complete stereoselectivity: two diastereoisomeric 2-methoxynorcaranes are formed, in which the deuterium atom occupies the endo-7 position. This means that the lack of strict stereoselectivity in the hydromethoxylation of compound I is related to the absence of selectivity at the stage of nucleophilic attack by the alcohol on intermediate 2-norcaranyl cation.1-Methyltricycloheptane III reacts with methanol in a similar way under metal complex catalysis: cleavage of the side C 2 -C 7 bond gives rise to two diastereoisomeric 2-methoxy-1-methylnorcaranes IVa and IVb (90%) together with 6-methoxy-1-methylcycloheptene (V) (10%) [5] (Scheme 1). The major products of acid-

Synthesis and Applications of Polysubstituted Bicyclo[1.1.0]butanes

“…BCB 2x , bearing a phenyl group at the bridgehead, underwent near-quantitative isomerization to cyclobutene 5 on treatment with a catalytic amount of HCl in chloroform, with excellent regioselectivity (98%, 20:1 rr ). , In contrast, reaction of methyl-bridgehead BCB 2c led exclusively to a single diastereomer of an α-chlorocyclopropane, tentatively assigned as 6 . That no cyclobutene or other product diastereomers were observed in this reaction would support a stereospecific ring-opening pathway that proceeds via a nonclassical carbocation ( 7 ) to give 6 , rather than stereoselective addition of chloride to a localized secondary cation …”