Addition methylene

Similarly, the direct [Eq. (11.24)] and photosensitized [Eq. (11.25)] decomposition of diazomethane in the presence of cyclohexene yielded product distributions indicative of greater selectivity in the triplet methylene addition ... 

Methylene addition to the C—C double bond of styrene (281) with [Fe(C5Me5)-(dppe) (=CH2)]" Bp4 (279) leading to phenylcyclopropane (282) proceeds in a CH2Cl2/MeCN-Bu4NPF6-Pt system at a potential of —0.6 V (SCF) via a radical intermediate (280) (Scheme 108) [417]. 

Treatment of the known nitroalkene 44 with the ylide indeed gave 41, but it was accompanied by a small proportion of the stereoisomer 4 (lA). Altljough the preparative yield was low (- <30%), the high diastereofacial selectivity of the methylene addition was remarkable. It becomes plausible on inspection of a molecular model, which points to hindered approach from one face, and unhindered approach from the other (Figure 7). 

Catalytic processes using diazo chemistry and stoichiometric methods with Fischer carbenes are complimentary for the introduction of a substituted carbene into a molecule. For methylene addition, however, there is no viable alternative to the modified Simmons-Smith reaction. Ring-closing metathesis, and its ROMP counterpart, have matured so fast that even now they rank among the most useful synthetic methodologies in organic chemistry. 

The Addition of Carbenes and Carbenoids to Double and Triple Bonds epr-Methylene-addition... 

The Formation of Epoxides from Aldehydes and Ketones (1 + 2)OC,CC-CKC/o-Methylene-addition... 

The question of retention of stereospecificity in methylene addition to olefins depends on competition between ring closure (reaction 91) and rotation about the carbon-carbon bond. 

The discrepancy may arise partly from the yet uncertain cis- to trn/i.v-1,2-dimethylcyclopropane ratio resulting from the triplet methylene addition to 2-butene and from the collision induced transition of methylene from singlet to triplet that may occur to a different degree depending on substrate molecules. 

Like the related fatty acid synthases (FASs), polyketide synthases (PKSs) are multifunctional enzymes that catalyze the decarboxylative (Claisen) condensation of simple carboxylic acids, activated as their coenzyme A (CoA) thioesters. While FASs typically use acetyl-CoA as the starter unit and malonyl-CoA as the extender unit, PKSs often employ acetyl- or propionyl-CoA to initiate biosynthesis, and malonyl-, methylmalonyl-, and occasionally ethylmalonyl-CoA or pro-pylmalonyl-CoA as a source of chain-extension units. After each condensation, FASs catalyze the full reduction of the P-ketothioester to a methylene by way of ketoreduction, dehydration, and enoyl reduction (Fig. 3). In contrast, PKSs shortcut the FAS pathway in one of two ways (Fig. 4). The aromatic PKSs (Fig. 4a) leave the P-keto groups substantially intact to produce aromatic products, while the modular PKSs (Fig. 4b) catalyze a variable extent of reduction to yield the so-called complex polyketides. In the latter case, reduction may not occur, or there may be formation of a P-hydroxy, double-bond, or fully saturated methylene additionally, the outcome may vary between different cycles of chain extension (Fig. 4b). This inherent variability in keto reduction, the greater variety of... 

HCN, 193, 442 de-conjugation, 159 dehydrogenation, 189 enamine formation, 185-186 enolisation, 156 ei seq. epoxidation, 18, 201, 442 formation, 109, 189, 248, 337 hydroboration. So Mannich reaction (aminometh-ylation), 172 methylation, 168-171 methylene addition, 202 oxidation, Baeyer-VUliger, 349, 445... 

Methylene ( CH2) addition to a double bond has been achieved by use of the Simmons-Smith reagent (CH2I2 and a zinc-copper couple) f 70J. The reaction gives a cyclopropane derivative by stereospecific cis addition, and in allylic or homo-allylic alcohols leads to methylene addition ci to the hydroxyl function. Examples in the steroid field include a s-methylene addition to the double bonds of the ga-hydrdxy-A dO), 3j8-hydroxy-AJ- and 3 a- and 3/9-hydroxy-A4 systems... 

C = O, conjugate addition, Grignard, 197 formation, 188-189 methylene addition, 202 reduction, 200... 

The Formation of Aziridines from Imines (1 + 2)AC,CC-cycto-Methylene-addition... 

FIGURE 2. Molecular orbital diagrams for carbonyl and methylene addition to cyclopropane. This orbitals are classified according to the reflections in the plane of the molecule and perpendicular to it (SS, SS, SA). Related orbitals of a given type are shown vertically under the symmetry heading... 

Ylide (1) adds selectively to the least hindered side of a spirocyclic cyclobutanone to afiord epoxide (10) as the sole product in 65% yield (Scheme 3). Where only steric factors are likely to be important, complementary stereochemical results are often possible by methylene addition to a carbonyl group or alternatively, by carrying out an epoxidation reaction on the derived alkene. In each case, the reagent approaches a double bond from the same stereochemical face. This point is nicely illustrated by the synthesis of epoxide (11). 

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