Preparation of vinylketene by 1,4-elimination. Cycloaddition and isomerization to form .alpha.-ethylidenecyclobutanones

Abstract

Vinylketene (1) was shown to result from triethylamine initiated 1,4-dehydrochlorination of trans-2-butenoyl chloride. In the presence of 1,3-cyclopentadiene a ,2 + T2 cycloaddition occurred to form adduct 2. With a trace of excess triethylamine 2 isomerized chiefly to a 73:27 mixture of the E and Z isomers 3 and 4, whose structures were securely assigned using lanthanide induced shift nuclear magnetic resonance techniques. The possible participation of ethylideneketene (CHaCH=C=C=0) was judged remote since triethylamine, 3-butenoyl chloride, and 1,3-cyclopentadiene gave an identical reaction mixture. With either isomeric acid chloride if less than 1.0 molar equiv of triethylamine was used a 60:01:39 mixture of 2:3:4 was formed, which upon addition of triethylamine equilibrated to the 73:27 mixture of 3 and 4. Ketenes (RR'C=C=0) react with conjugated dienes in a highly perispecific and regiospecific manner to form 3-• vinylcyclobutanones.4 Since the olefinic component reacts suprafacially, the fusion geometry is consistently cis. When the two ketene substituents differ in steric bulk the larger one tends to assume the more hindered position in the product.5,6 Ketene cycloadditions represent one of the few synthetic approaches to four-membered rings,7 and also serve as sensitive mechanistic probes of the rare and difficult T2S + T2a allowed concerted8 pathway. A review of the available evi-dence4 and recent theoretical analyses9,10 indicate that both concerted and nonconcerted routes may be traversed, depending on the steric and electronic characteristics of ketene and ketenophile.

Acid-catalyzed rearrangement of cyclobutanones. VI. Syntheses of chrysenes and steroid-like substances

“…The structures are assigned based on spectral and analytical data and by analogy with other ketene 1,3-diene cycloaddition products (8). The regiospecificity of cycloadditions of ketenes is well established (9, 10); however, the stereochemistry of the naphthyl group with respect to the ring junction has not been unequivocally defined, although there is evidence that when two ketene substituents differ in steric bulk the larger one tends to occupy the more hindered endo position in the product (1 l,12).…”

Coupling of acyl chlorides with triarylbismuths catalyzed by palladium bipyridyl complex anchored on nanosized MCM-41: A recyclable and atom-efficient catalytic process for the synthesis of diaryl and alkyl aryl ketones

“…However, when an alkyl acyl chloride such as valeroyl chloride was employed, no desired product was observed. Several reports show that Et 3 N reacts with linear aliphatic acyl chlorides to form ketenes and subsequent [2 + 2] cycloaddition gave oxetanes [61][62][63]. Replacing Et 3 N with bulky diisopropylethylamine as a base avoided the side reaction and the desired alkyl aryl ketones were obtained in good yields (Entries 7-9).…”

2‐Vinylcyclobutanones by Cycloaddition of Vinylketenes to Simple Olefins

“…The two methods for the preparation of IV are: 1 ) rearrangement of 1-functionalized cyclopropyl-vinyl-carbinols [5] and 2) the cycloaddition of vinylketenes 11 to olefins 111 (Scheme 1). While Method 2 is effective with activated olefins, such as conjugated dienes [I-31 [6] [7] fulvenes [l], enarnines [8] and enolethers [91 [lo], it has so far given only low yields with simple olefins 111 (cJ [3]). chloride l a as a standard since it gave reasonable yields still responsive to structural features of the olefin.…”