Exo-methylene

The base catalyzed rearrangement of a monotosylated 1,2-diol on alumina, followed by immediate condensation of the sensitive ketone with methylenetriphenylphosphorane, gave the exo-methylene compound below (G. Btlchi, 1966B). 

Various terminal allylic compounds are converted into l-alkenes at room temperature[362]. Regioselective hydrogenolysis with formate is used for the formation of an exo-methylene group from cyclic allylic compounds by the formal anti thermodynamic isomerization of internal double bonds to the exocyclic position[380]. Selective conversion of myrtenyl formate (579) into /9-pinene is an example. The allylic sulfone 580 and the allylic nitro compound... 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

Grubbs et al. reported that the ruthenium-catalyzed RCM of a conjugated diene proceeds in such a way that the less hindered olefin moieties participate in the reaction. Consequently, RCM of 115 gives exo-methylene compound 116, and not exo-vinyl compound 117 (Scheme 24) [105]. This regioselectivity is complementary to that observed for enyne metathesis of 118, which gives exclusively 117 (Scheme 24) [106a]. 

Beim Cephalosporin sowie 7-Amino-cephalosporinsaurc wird unter C-O-Spaltung und Allyl-Verschiebung eine exo-Methylen-Gruppe erhalten1 ... 

Reactive polyesters were enzymatically synthesized. Lipase catalysis chemoselecfively induced the ring-opening polymerization of a lactone having exo-methylene group to produce a polyester having the reactive exo-methylene group in the main chain (Scheme 16). This is in contrast to the anionic... 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

The study on 2,7-di-rerf-butylthiepin has recently been extended to explore more simply substituted thiepins 58). The palladium-catalyzed reaction of the diazo compound 107 lacking a 4-methyl substituent gives exclusively the exo-methylene compound 108 whereas the acid-catalyzed reaction of the same precursor 107 resulted in the formation of 2,7-di-/er/-butyl-4-ethoxycarbonylthiepin (109)58). Due to the substantial thermal stability of 109 it is possible to transform the ethoxy-carbonyl group into the hydroxymethyl (110), trimethylsilyloxymethyl (111) and formyl group (112)58). 

Recently, a new multicomponent condensation strategy for the stereocontrolled synthesis of polysubstituted tetrahydropyran derivatives was re-published by the Marko group, employing an ene reaction combined with an intramolecular Sakurai cyclization (IMSC) (Scheme 1.14) [14]. The initial step is an Et2AlCl-promoted ene reaction between allylsilane 1-50 and an aldehyde to afford the (Z)-homoallylic alcohol 1-51, with good control of the geometry of the double bond. Subsequent Lewis acid-media ted condensation of 1-51 with another equivalent of an aldehyde provided the polysubstituted exo-methylene tetrahydropyran 1-53 stereoselectively and... 

The substitution of the exo-methylene hydrogen atoms of MCP with halogens seems to favor the [2 + 2] cycloaddition reaction by stabilizing the intermediate diradical. Indeed, chloromethylenecyclopropane (96) reacts with acrylonitrile (519) to give a diastereomeric mixture of spirohexanes in good yield (Table 41, entry 2) [27], but was unreactive towards styrene and ds-stilbene. Anyway, it reacted with dienes (2,3-dimethylbutadiene, cyclopentadiene, cyc-lohexadiene, furan) exclusively in a [4 + 2] fashion (see Sect. 2.1.1) [27], while its... 

Epoxides can also be reductively opened to form a radical. An example of an intramolecular cyclization of such a radical has recently been reported <06TL7755>. Treatment of 40 with Cp2TiCl generates an intermediate alkoxy radical, which then adds to the carbonyl of the formate ester. The product, 41, is formed as a 2 1 mixture of isomers at the anomeric carbon. This reaction is one of the first examples of a radical addition to an ester. The major byproduct of this reaction is the exo-methylene compound, 42, arising from a P-hydrogen elimination. 

More recently, Yang et al. [123] have examined a new approach in which a reactive functional group was introduced into polyolefins using methylenecyclopropane (Scheme 27). Thus, ethylene (1.0 atm) was copolymerized with methylenecyclopropane (0.25-2.5 ml) using [LnH(C5Me5)2]2 (Ln = Sm, Lu) in toluene at 25 °C. It was shown that 10-65 units of exo-methylenes were incorporated per 1000-CH2- units and the resulting polymer had an Mw of 66-92 x 103, yet its Mw/Mn was > 4. 

The reductive cyclization of enynes has been used to prepare exo-methylene-cycloalkanes. Two systems have proven successful in this transformation, namely PMHS/Pd2(dba)3 CHCl3245 (Eq. 102) and Et3SiH/Pd(dppe)Cl2/HC02H (Eq. 103).246... 

Numerous heteroradialenes, in which the exo-methylene groups are replaced by oxygen, nitrogen or sulfur atoms, have also been synthesized and studied because of their interesting electrical and magnetic properties. Heteroradialenes are, however, not included in this review. 

Except for the still thermochemically uninvestigated 1,2-bismethylenecyclopropane44, 57, all bismethylenecycloalkanes can further be divided into two categories—those in which the exo-methylene groups are on adjacent carbons and those further apart. The two isomeric bismethylenecyclobutanes have been studied. Roth presents an enthalpy of formation for the 1,2-isomer, 58, of 204.2 kJmol-1. In the absence of any additional strain-induced destabilization or conjugative/delocalization-induced stabilization, we would expect the disproportionation reaction 27 of methylenecyclobutane (59)... 

VII. CONJUGATED SPECIES WITH EXO-METHYLENE GROUPS FULVENES, ISOTOLUENES, XYLYLENES AND RELATED SPECIES... 

The r2 for this line is 0.9995 with a standard deviation of ca 0.9 kJmol 1 (Figure 1). The deviation for R1 = R2 = Ph (129) is some 43 k,I mol 1 below the line. If the exo-methylene/ring bond is quite polar and resonance structures 128 are significant (130 is easily ignorable), then AHf (127, R1, R2) and A//f(0=CR1R2) would be more likely to be linearly related. Another nearly perfect line, that of carbonyls vs fulvenes, can be drawn through the (H,H), (H,Me) and (Me,Me) points (equation 36). 

The parent triafulvene, 104, is the sole representative of this hydrocarbon class for which there is a suggested enthalpy of formation75, namely 423 kJ mol 1. If the conjugative interactions of the exo-methylene with cyclopropene and cyclopentadiene were the same, then equation 39 would be thermoneutral. 

Tributyl tin radical mediated cyclization of the glucose derived exo-methylene furanose derivatives led to highly functionalized cA-fused bicyclic ethers. The product could subsequently be transformed into optically active tricyclic nucleoside analogue or oxepine derivative (Fig. 51).67... 

Synthesis from exo-glycals. Several routes to C-l glycals from 1-exo-methylene pyranoses and furanoses have been described recently. 

---