Cyclopentenyl cyclopropanations

Silyl enol ethers can also be used in the cyclopropanation reaction. Reissig showed that the reaction between methyl diazoacetate 53 and various enol ethers 52a-c using bu-box ligand 3 proceeded in moderate yields, as shown in Table 9.5 (Fig. 9.17fl), with trans/cis ratios up to 97 3 and ee between 32 and 49%. Pfaltz showed that cyclic enol ethers can be used as well." Cyclopentenyl enol ether 55 proceeded with methyl diazoacetate 53 and bu-box ligand 3 to afford the cyclopropanation products in 56% yield, a trans/cis ratio of 27 73, trans ee of 87% and cis ee of 92% (Fig. 9.11b, p. 544). 

A ring-opening has also been seen, when the relief of strain in a cyclopropane makes it thermodynamically favourable—the cyclopentenyl anion 4.100 opens to the pentadienyl anion 4.101. This reaction had no option but to be disrotatory with the two hydrogen atoms moving outwards 4,98, since a trans double bond is impossible in the 6-membered ring. 

This collection begins with a series of three procedures illustrating important new methods for preparation of enantiomerically pure substances via asymmetric catalysis. The preparation of 3-[(1S)-1,2-DIHYDROXYETHYL]-1,5-DIHYDRO-3H-2.4-BENZODIOXEPINE describes, in detail, the use of dihydroquinidine 9-0-(9 -phenanthryl) ether as a chiral ligand in the asymmetric dihydroxylation reaction which is broadly applicable for the preparation of chiral dlols from monosubstituted olefins. The product, an acetal of (S)-glyceralcfehyde, is itself a potentially valuable synthetic intermediate. The assembly of a chiral rhodium catalyst from methyl 2-pyrrolidone 5(R)-carboxylate and its use in the intramolecular asymmetric cyclopropanation of an allyl diazoacetate is illustrated in the preparation of (1R.5S)-()-6,6-DIMETHYL-3-OXABICYCLO[3.1. OJHEXAN-2-ONE. Another important general method for asymmetric synthesis involves the desymmetrization of bifunctional meso compounds as is described for the enantioselective enzymatic hydrolysis of cis-3,5-diacetoxycyclopentene to (1R,4S)-(+)-4-HYDROXY-2-CYCLOPENTENYL ACETATE. This intermediate is especially valuable as a precursor of both antipodes (4R) (+)- and (4S)-(-)-tert-BUTYLDIMETHYLSILOXY-2-CYCLOPENTEN-1-ONE, important intermediates in the synthesis of enantiomerically pure prostanoid derivatives and other classes of natural substances, whose preparation is detailed in accompanying procedures. 

Cyclopropan l-(l-Cyclopentenyl)-l-piperidino- E17b, 1586 (OSiR3 -+ Cyclopen tenyl)... 

Cyclopropan l-(l-Cyclopentenyl)-2-ethenyl-l-trimethylsilyloxy-El7a, 312 (cycl. 1,3-Dien +... 

Cyclopentanecarboxaldehyde, 493 Cyclopentanecarboxylic add, 494,495 Cydopentanone, 139, 214,445, 551 Cydopentene, 319 Cydopentene epoxide, 404 2-Cyclopentenone, 158, 227, 262, 511 2-Cyclopentenones, 218 Cyclopentenone thioketals, 218 1-Cyclopentenyl isobutyl ketone, 512 Cyclopentyne, 209 Cyclopropanation, 153 Cyclopropanecarboxaldehyde, 43,44, 520-521... 

The photochemical reaction of methanethiol with bicyclo[3.1.0]hex-2-ene (43) afforded a pair of isomeric cyclopentenyl sulfides 44 via /i-scission of the outer cyclopropane bond in the adduct radical. At low thiol concentrations these were the major products unrearranged compounds became important at higher thiol concentrations. 

The opposite is valid in the NMR spectrum of ion 455 the signal of the methylene carbon C is 80 ppm higher than that of (C ). The constants for the cyclopropane carbons of ion 491 are essentially larger than their counterparts for ion 455. Thus, ion 491 fits better to a cyclopentenyl ion with a significant delocalization of the positive charge into the condensed cyclopropane ring with a sufficiently strong C —C bond. 

Figure 5.1 Structures of naturally occurring cyclic fatty acids. I, Cyclopropane acids a, 9,10-methylenehexadecanoic acid b, 11,12-methyleneoctadecanoic (lactobacillic) acid c, 8,9-methy-leneheptadecanoic (dihydromalvalic) acid d, 9,10-methyleneoctadecanoic (dihydrosterculic) acid. II, Mycolic (2-alkyl-3-hydroxy) acid. Ill, Cyclopropene acids a, 9,10-methyleneoctadec-9-enoic (sterculic) acid b, 8,9-methyleneheptadec-8-enoic (malvalic) acid. IV, Cyclopentenyl acids a, ll-cyclopent-2-enyl-undecanoic (hydnocarpic) acid b, 13-cyclopent-2-enyl-tridecanoic (chaulmoogric) acid c, 13-cyclopent-2-enyl-tridec-6-enoic (gorlic) acid. V a, 11-cyclohexyl-undecanoic acid b, 11-cycloheptylundecanoic acid.

---