Cyclopropanols, intermediate

The reaction may include a cyclopropanol intermediate derived from an anion radical, as seen in the reduction of cyclohexenones under Clemmensen conditions to afford ring-contracted cyclopentanones along with cyclohexanone derivatives.Thus the diastereomeric cyclopropanol acetates (12) and (13) can be obtained in different ratios from both (10) and (11) (12/13 = 3 from 10, 12/13 > 100 from 11 Scheme 6). ... 

An interesting variation has recently been described using 1,3-dichloroacetone for the preparation of alkyl ketones via cyclopropanol intermediates (equation 68). ... 

Homologation a-Trimethylstannylmethyl-p-ketoesters are converted to 7-ketoesters in the presence of NbClj, presumably via cyclopropanol intermediates (6 examples, 34-88%). The tributylstannylmethyl analogues give inferior results. 

Table 11.5. Substituted cyclopropanols 31 from carboxylic acid esters and alkenes via ligand-exchanged titanium intermediates generated from Grignard reagents and XTi(OiPr)3 (X = OiPr, Cl, Me).

In the hydroxycyclopropanation of alkenes, esters may be more reactive than N,N-dialkylcarboxamides, as is illustrated by the exclusive formation of the disubstituted cyclopropanol 75 from the succinic acid monoester monoamide 73 (Scheme 11.21) [91]. However, the reactivities of both ester- as well as amide-carbonyl groups can be significantly influenced by the steric bulk around them [81,91]. Thus, in intermolecular competitions for reaction with the titanacydopropane intermediate derived from an alkylmagnesium halide and titanium tetraisopropoxide or methyltitanium triisoprop-oxide, between N,N-dibenzylformamide (48) and tert-butyl acetate (76) as well as between N,N-dibenzylacetamide (78) and tert-butyl acetate (76), the amide won in both cases and only the corresponding cyclopropylamines 77 and 79, respectively, were obtained (Scheme 11.21) [62,119]. 

Thus, in a novel synthesis of hypoglycine A (96), hydroxycyclopropanation of ethenyl-acetaldehyde diethyl acetal (93) followed by formal dehydration of the cyclopropanol 94 via its tosylate intermediate gave the methylenecyclopropane species 95, a key precursor to the target amino acid (Scheme 11.26) [92]. 

Hydroxymethylenecyclopropanols (340) have been shown" to be intermediates in the photochemical rearrangement of a, -unsaturated carbonyl compounds (339) to 1,4-dicarbonyl compounds (341). The products are eventually obtained by double tautomerization of the enol and cyclopropanol portions of (340). 

Such disjoint molecules may serve as synthetic intermediates. Thus a [3 + 2] annulation method exploits the facile ring opening of cyclopropanols and the transfer of the donor site to the p carbon of the resulting ketones, the nascent carbanions may... 

Condensation of /3-(hydroxyalkyl)cyclopropanols with aldehydes, followed by rearrangement of acetals formed (Scheme 27), is proposed as simple and convenient method for the synthesis of c -2,7-disubstituted oxepan-4-ones <20050L515>. Though the intermediate acetal could be isolated in some cases, the reaction is usually carried out as one-pot procedure with sequential addition of two Lewis acids, Al(OTf)2 and TiCl4. The overall yields range from 50% to 70%. 

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