Cyclobutane from lactones

Ketene dimerization is the principal synthetic route to 4-methylene-2-oxetanones. This reaction proceeds very satisfactorily for ketene and methylketene, but disubstituted ketenes dimerize only to cyclobutane-1,3-diones. The cycloaddition reaction of r-butylcyanoketene to ketene and to methylketene gives a-cyanoalkylidene-/3-lactones in about 40% yield in addition to the cyclobutane-1,3-dione dimer of f-butylcyanoketene. A mechanism has been proposed for the formation of both types of dimers from a common zwitterionic intermediate (equation 111), with the relative amount of each product determined by the configurational equilibrium of the intermediate (80JOC4483, 75JOC3417). 

Irradiation of lactone 131 with ( )-anethole in benzene gave cyclobutane 132 together withp-methoxybenzaldehyde [215] (Scheme 43). Similar irradiation of lactone 133 gave both cyclobutane 134 and an olefin 136 derived from an oxe-tane precursor 135. 

The fadal diastereoselectivity of intermolecular cyclopentenone [2 + 2]-photocy-cloaddition reactions is predictable if the cyclopentenone or a cyclic alkene reaction partner is chiral. Addition occurs from the more accessible side, and good stereocontrol can be expected if the stereogenic center is located at the a-position to the double bond. In their total synthesis of ( )-kelsoene (11), Piers et al. [22] utilized cyclopentenone 9 in the [2 + 2]-photocycloaddition to ethylene (Scheme 6.5). The cyclobutane 10 was obtained as a single diastereoisomer. In a similar fashion, Mehta et al. have frequently employed the fact that an approach to diquinane-type cis-bicydo [3.3.0]octenones occurs from the more accessible convex face. Applications can be found in the syntheses of (+)-kelsoene [23], (—)-sulcatine G [24], and ( )-merri-lactone A [25]. 

Liu has reported recently a new Co2(CO)8-mediated tandem [5+ 1]/ [2 + 2+ 1]-cycloaddition reaction that gives tricyclic 5-lactones from cis-epoxy enynes 186. This process possibly involves an initial opening of the epoxide in the cobalt hexacarbonyl complex 187 to from the complexed al-lene 189 via 188. Further coordination of the tethered olefin leads to 190 and further oxidative cyclization gives cobaltacycle 191 which inserts CO leading to the final compound 192. When performing the reaction under N2,191 suffers a reductive elimination to give cyclobutane 193 (Scheme 55) [172],... 

Subsequently, the absolute configuration of the natural lineatin was determined as (lR,4S,5R,7R)-77 by our second synthesis, as shown in Figure 4.16.35 The first step was the cycloaddition of dichloroketene to isoprene to construct a cyclobutane ring. The symmetrical cyclobutanone A was then converted to ( )-bicyclic lactone B. Enantiomer separation (optical resolution) of ( )-B was executed as follows. Reaction of ( )-B with the resolving agent C (derived from chrysanthemic acid) yielded a diastereomeric mixture... 

The photochemical [2+2]cycloaddition of alkenes to coumarins provides a mixture of the exo- and e <7o-cyclobuta[c]coumarins. Treatment with CH2=SOMe2 opens both the lactone and cyclobutane rings and both diastereomers are considered to give the same quinone methide from which a single dibenzofuran-4-ol stereoisomer results <04SL1897>. 

Jeffares and McMurray have isolated the cyclobutane lactones (78) in low yield from the photochemical rearrangement of the 6-hydroxycyclohexenones (79). These are the first examples of products derived from cyclohexenones by an a-cleavage process. Irradiation in the presence of triplet quenchers such as ferrocene improves the yields somewhat. 

Both enol esters and enol ethers (vinylogous esters) undergo similar intramolecular photocycloaddtions, but enol esters have advantage over enol ethers in terms of fragmentation process. Fragmentation of the cyclobutane photoadducts from enol esters is conveniently carried out under basic conditions, whereas that from enol ethers often requires additional operation to convert ether to ester or lactone (vide infra). For this reason, enol esters are more widely used than enol ethers. 

Thionation of Carbonyl Compounds. Diaryl thioketones and 2,4-bis(diphenylmethylene)cyclobutane-l,3-dithione were obtained on reaction of the corresponding ketones with DR. On the other hand, JR was utilized to thionate selectively the carbonyl group of benzil monophenylimine. If carboxylic esters or lactones are reacted with DR, dithiocarboxylic esters or dithiolac-tones are formed, i.e. the carbonyl group is thionated and the alkoxy group is substituted by an aUcylthio group. Bis(5-methyl-2-thienyl) disulfide is produced in this way from a butenolide (eqS). ... 

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