Methylenomycin synthesis

Methylenomycins.1 The cyclopentannelation reaction (12,310) has been modified to provide a general synthesis of methylenomycins. Thus, the adduct (2) of a-lithio-a-(methoxymethyl)allene (1) with 3-methyl-3-butene-2-one cyclizes to methylenomycin B (3) in the presence of trifluoroacetic anhydride and 2,6-lutidine. 

The only limitation noted in the synthesis of related methylenomycins is that only the adducts of (E)-enones undergo cyclization. 

Moreto and coworkers have made improvements to the Chiusoli reaction, the Ni(CO)4-mediated carbonylation cyclization of allyl halides and alkynes, by conducting it in methanol403. It has subsequently been applied in the synthesis of methylenomycin B, in an intramolecular sense to provide bicyclo[3.3.0]octenones, and in intermolecular cases to form both fused bicyclic cyclopentenones and spirocyclopentenones (equations 203 and 204)403-405. 

Assereg, J.H., Glase, S.A., and Welch, S.C. 1987. 3-Chloro-2-[(diethoxyphosphoryl)oxy]-l-propene a new reagent for a one-pot cyclopentenone annelation. Synthesis of desoxyal-lethrolone, cw-jasmone, and methylenomycin B. Journal of Organic Chemistry, 52(8) 1440-50. 

In a synthesis of methylenomycin B (5), the methylthiomethylene group, the precursor of the cjco-methylene group, was introduced at an early stage by alkylation of a p-keto... 

A recent synthesis of methylenomycin-B (74), based on these observations, features an allylation/car-bonylation (69) - (70) followed by a C-1—C-2 bond/nickel acyl insertion (70) -> (71) and a final meth-oxycarbonylation (71) -> (72). Thus 2-butyn-l-ol (1,2-dialkylacetylenes are inert) afforded in one synthetic operation a 1 4 mixture (78%) of regioisomeric cyclopentenones (72) and (73) which was converted to the antibiotic (74) (Scheme 16). ... 

Mikolajczyk, M., and Balczewski, P., A new, total synthesis of methylenomycin B using organic sulfur and phosphorus reagents. Synthesis, 691, 1984. 

A recent synthesis of methylenomycin-B (74), based on these observations, features an allylation/car-bonylation (69) (70) followed by a C-1—C-2 bond/nickel acyl insertion (70) - (71) and a final meth-... 

Numerous synthetic applications of the inteimolecular Pauson-Khand reaction have been reported. Pauson has reported a number of very direct tqrplications of cycloadditions of ethylene in the synthesis of prostanoids and jasmone analogs (e.g. equations 15 and 16). - This is a reliable entry to 2-sub-stituted cyclopentenones. The suitability of cyclopentene and dihydrofuran as substrates has permitted the extension of this work to the preparation of still further varieties of prostaglandin analogs (e.g. equations 27 and 51). Simple 4,5-disubstituted 2-cyclopentenones are not as directly accessible, but may be prepared from the cycloaddition products of norbomadiene (equation 45). A sequence of conjugate addition followed by retro-Diels-Alder reaction affords the product (Scheme 5). Dihydrofuran cycloadditions have been used by Billington in the syntheses of the antibiotic methylenomycin B (Scheme 6), as well as cyclomethylenomycin A (synthetic precursor to the antibiotic methylenomycin A), cyclosarko-mycin (precursor to the antitumor agent sarkomycin) ° and the iridoid Jq>anese hop ether. ... 

The synthesis of the antibiotic methylenomycin A 114 is instructive on the choice between modern and traditional methods. The key intermediate 116 has been converted into methylenomycin by several groups whose strategy differs only in the synthesis of 116. The Pauson-Khand disconnection is simplicity itself, requiring the symmetrical unsaturated ether 117 and symmetrical butyne. 

Unsaturated acyl chlorides react with substituted alkynes to give mixtures of linear and cyclopentenyl products, the latter being formed in greater yield at higher temperatures (Scheme 22). ° This offers a convenient access to a variety of S-chlorocyclopentenones bearing substituents at the 4- and S-positions, with control over the location of the double bond. Application of this method includes a short synthesis of the antibiotic methylenomycin B (17 Scheme 23). ° ... 

Several other examples of stereoselective synthesis of polycyclic ketones, via carbonylative [2 + 2 + 1] cycloaddition of organopalladium compounds derived from norbornene and norbor-nadiene, have been reported53,54,121. These reactions are useful in the synthesis of cyclopen-tanoid compounds, such as dihydrojasmone54 The nickel-catalyzed version of this method was used in a total synthesis of methylenomycin B55 and in the stereoselective synthesis of bicy-clo[3.3.0]oct-l-en-3-one derivatives56. Thus, (E)- or (Z)-9-bromo-l-methoxy-7-nonen-2-yne (5), upon intramolecular carbonylative cyclization promoted by tetracarbonyl nickel, afford the same stereoisomer of methyl 1,2,4,5.6,6a-hexahydro-3-methoxymethyl-2-oxo-l-pentaleneac-etate (6) in 43-50% yield with a relative trans configuration of the H-l and H-6a protons. 

Keto ester (9) was needed for a synthesis of the antibiotic methylenomycin (8). The first disconnection should be at the a,/3 double bond and this reveals an obvious 1,4-disconnection (10) with the activating group C02Et already present. 

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