Cyclopropene, 3,3-dimethoxy

Carboxy-l-methyl- E17a, 836 ( -CH2 — COOR + Oxiran/NaH) Cyclopropen 3,3-Dimethoxy- E17d, 2723 [3-Ct — 1 -Cl — 2,2-(OR)2 — propan/KNH2]... 

Cyclopropan 2,3-Dimethoxycarbo-nyl-1 -(2,2-dimethoxy-ethenyl)-E17a, 238 (subst. Cyclopropen 4-ROOC-en-COOR)... 

The reaction between dimethyl 2-bromo-l-phenylpropylidenemalonate and sodium methoxide in methanol affords a complex reaction mixture consisting of dimethyl 2-methyl-3-phenyl-2-cyclopropene-l,l-dicarboxylate (303,19%), dimethyl (2,2-dimethoxy-l-methyl-2-phenylethyl)malonate (304, 4%), dimethyl 2-methoxy-l-phenylpropyl-idenemalonate (305, 11%), dimethyl 2-bromo-l-phenyl-1-propenylmalonate (306 -isomer, 10% Z-isomer, 13%) and dimethyl (l,2-dimethoxy-l-phenylpropyl)malonate... 

Remarkably, with 3,3-dimethoxycyclopropene, the stereochemical outcome of the [2 4-1] cycloaddition reaction with dialkyl fumarate and maleate is different to that observed for the other 3,3-disubstituted cyclopropenes (Table 3). With the 3,3-dimethoxy derivative only the thermodynamically more stable 1,2-tra s-dialkoxycarbonylcyclopropanes are formed. This can be taken as an indication for a stepwise addition process with an intermediate of highly zwit-terionic character (structure 29 B or 29 C) that can undergo rotational isomerization to form the thermodynamically most stable product, i.e. the tranx-configurated diester. 

When isomerization of the double bond of the intermediate cyclopropene into the exo methylene position is energetically unfavored, double elimination followed by double addition of base can occur. Thus, reaction of (cu/tra i )-l,l-dichloro-2-methyl-3-vinylcyclopropane (9) with potassium terf-butoxide plus methoxide in dimethyl sulfoxide gave a mixture of cis- and trans-, -dimethoxy-2-methyl-3-vinylcyclopropane (10). Both double bonds formed via elimination appear to be conjugated with the vinyl group and do not migrate to the exo position. The similar reaction of l,l-dichloro-2-[( )-propenyl]cyclopropane (11) gave a small amount of 1-/ert-butoxy-2-(prop-2-enylidene)cyclopropane (13) in addition to products of double addition. Without added methoxide, 13 was the only product isolated. ... 

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