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Abstract
In the temperature range 244-268$\sp\circ$C, the diastereomers of $cis$-1-(1-methoxyethyl)-2-vinylcyclobutane (1$RS,2RS,1\sp\prime SR)$-110 and (1$SR,2SR,1\sp\prime SR)$-111 each undergo a homo (1,5) -sigmatropic hydrogen shift (retro ene) reaction (roughly 18% of the total product in both cases), in addition to four other unimolecular processes. These others are: (1) retro (2 + 2) ring fragmentation; (2) epimerization of one or (3) both ring stereogenic centers; and (4) sigmatropic (1,3) -rearrangement. In the retro ene reaction 110 yields (2$E,6Z$)-2-methoxyocta-2,6-diene 131, while 111 gives the (2$Z,6Z$)-isomer 130 with stereospecificities of 95% and 91%, respectively. In both cases, the minor isomeric retro ene products are largely attributable to non-concerted, or secondary rearrangements. Consequently, the stereospecificity of the predominant, potentially concerted retro ene reaction is $\geq$99% (from 110) or $\geq$96% (from 111). Control experiments demonstrate product stability to the reaction conditions, and the absence of a stereoelectronic directing effect by the methoxy group.
At 239$\sp\circ$C, enantiomerically enriched ($-$)-(1$R,2R,1\sp\prime S)$-1-(1-methoxyethyl)-2-(2-deuterio-1$E $-propenyl)cyclobutane 108 rearranges to give approximately 6% retro ene reaction. This retro ene reaction yields (8$S$)-(2$E,6Z$)-deuterio-2-methoxynona-2,6-diene 171 with $\ge$93% stereospecificity with respect to olefin stereochemistry. Product 171 was degraded to (2$S$)-2-$d$-propanoic acid 83, whose enantiomeric excess was determined using a standard method. The minimum transfer of stereogenicity in the formation of (8$S$)- 171 lies in the range 68.9-80.5%. The hydrogen transfer leading to 171 is thus suprafacial, and thermally allowed. These results strongly suggest a concerted retro ene reaction, with a transition state characterized by maximal orbital overlap between the olefinic $\pi$-bond, the breaking C-H bond, and the breaking C-C ring bond.
