3-Butynone

Butynone 93 and tetrasubstituted allene 94 combine to form acetyl cyclopenta-diene 95 (Eq. 13.30) [32]. The annulation does not proceed in satisfactory yield in the case of unsubstituted trimethylsilyl allene 96 (Eq. 13.31) [32]. This is presumably due to diminished stabilization of the cationic intermediates in the case of 96, which allows competing reaction pathways to erode the yield. 

It is noteworthy that the ene product 227 is obtained exclusively when allylsilane 226 is treated with butynone in the presence of Znl2 catalyst and molecular sieve (ms) 4A(equation 185)331. Methyl vinyl ketone behaves similarly. High enantioselectivity has been observed in product 228 when triphenylallylsilane is coupled with methyl glyoxylate in the presence of (R)-(BINOL)TiCl2 catalyst (equation 186)332. In addition to [2 + 2] cycloaddition, TCNE undergoes a regiospecific ene reaction with y-alkyl substituted allylsilanes to yield the substituted olefins 229 (equation 187)333. 

Reductive cyclization of butynoates with PPhs opens an easy access to 2,3,4-trisubstituted furans <2004JA4118>. Allenones of type 17 are proposed to be intermediates in this reaction (Scheme 21). One example for the reaction of a butynone is also given. 

The bis(hydroalkoxylation) reaction of butynone 11 with ethanediol and 5% DMAP gives the valuable synthetic intermediate 12 in 81% yield <05CC227> and a range of ring-fused... 

The chiral steroidal cyclopentadiene 1 reacts with propargylic aldehyde (2a) or butynone (2b) yielding in each case, a single cycloadduct in 94 and 91 % yield, respectively20. 

The formation of oxetanes bearing side chains containing unsaturation has been reported78. It should be noted that irradiation of unprotected acetylene terminal 2-butynone gave only low isolated yields of an oxetane. 

The monoaddition of bulky dienophiles such as ethylenetetracarboni-tirle (TCNE) to 10 are generally exo face selective (e.g., giving adduct 57). However, less bulky dienophiles such as butynone or methyl vinyl ketone prefer, for stereoelectronic reasons, to add onto the endo face of 10 giving monoadducts 58 and 59, respectively [201]. 

Figure 10.30 Matching MS of two radically diffiaient compound structures one aligned and displayed inverted below the other. (Note the lower compound name is incorrect in the spectmm. It is 2-Butynone, 1-acetyM-[1-piperidyI]-.) (From the NIST Mass Spectrometry Data Center accessed via http // webbook.nist.gov. 2011 U.S. Secretary of Commerce for the United States of America. All rights...

The use of other electrophiles in the Danheiser annulation for the formation of cyclopentenes is relatively unexplored. Ynones have been reported to imdergo cycloaddition, although examples are limited. Thus treatment of butynone 64 with silyl allene 47 under the usual conditions delivered cyclopentadiene 65 in 53% yield. Danheiser has reported that nitroalkenes do not provide annulation products furnishing instead acyclic alkynes. For example, treatment of nitroalkene 66 with silylallene 34 under standard conditions gives alkyne 67 rather than the cyclopentene 68. afi-Unsaturated aldehydes have been reported to be problematic, failing to provide isolable cyclopentene products. Treatment of methacrolein (69) with silylallene 34 under the standard conditions produced only a complex mixture of unidentified products. 

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