Propargylic alcohols allene synthesis

Enyne metathesis starting either from acetylenic boronates and homoallylic alcohols [104a,c] or from propargyl alcohols and allylboronates [104b] has recently been described. The resulting boronated dienes can be converted to allenes or cycloaddition products. The cross metathesis of vinylcyclopropyl-boronates directed toward the total synthesis of natural products has very recently been investigated by Pietruszka et al. [104d]. 

Jin and Weinreb reported the enantioselective total synthesis of 5,11-methano-morphanthridine Amaryllidaceae alkaloids via ethynylation of a chiral aldehyde followed by allenylsilane cyclization (Scheme 4.6) [10]. Addition of ethynylmagnesium bromide to 27 produced a 2 1 mixture of (S)- and (R)-propargyl alcohols 28. Both of these isomers were separately converted into the desired same acetate 28 by acetylation or Mitsunobu inversion reaction. After the reaction of 28 with a silyl cuprate, the resulting allene 29 was then converted into (-)-coccinine 31 via an allenylsilane cyclization. 

Carreira and co-workers developed a highly efficient enantioselective addition of terminal alkynes to aldehydes giving propargyl alcohols by the mediation of zinc tri-flate and N-methylephedrine [17]. This reaction serves as a convenient and powerful synthetic route to a wide variety of enantioenriched allenes via propargyl alcohols. Dieter and Yu applied this alkynylation to the asymmetric synthesis of allenes (Scheme 4.12) [18]. Reaction of phenylacetylene with isobutyraldehyde afforded the propargyl alcohol in 80% yield with 99% ee, which was mesylated to 49 in quantitative yield. Reaction of 49 with the cyanocuprate 50 afforded the desired allene 51 with 83% ee. 

In 1963, an asymmetric synthesis of chloroallenes was reported by the SNi reaction of propargyl alcohols with thionyl chloride [34]. Since then, rearrangement of pro-pargylic precursors has been one of the most useful methodologies for the synthesis of allenes [35]. Treatment of 84, obtained by asymmetric reduction with LiAlH4-Dar-von alcohol complex, with thionyl bromide gave 86 as the major product via 85 (Scheme 4.21) [36],... 

A stereoselective synthesis of the enantiomerically enriched allenic hydrocarbons was described in 2001 (Scheme 18.11) [37]. For example, hydrostannylation of the chiral propargylic alcohol 28 (obtained with 82% ee by enantioselective reduction of... 

Scheme 20.5 One-step synthesis of enyne-allenes from enediynyl propargylic alcohols.

The reaction sequence outlined in Scheme 20.30 for the preparation of the chlorinated enyne-allenes was successfully adopted for the synthesis of the C44H26 hydrocarbon 251 having a carbon framework represented on the surface of C60 (Scheme 20.50) [83]. Condensation of the monoketal of acenaphthenequinone (243) with the lithium acetylide 101 afforded the propargylic alcohol 244. On exposure to thionyl chloride, 244 underwent a cascade sequence of reactions as described in Scheme 20.30 to furnish the chloride 248. Reduction followed by deprotection produced 250 to allow a repeat of condensation followed by the cascade transformation and reduction leading to 251. 

Allenic alcohols T reatment of the propargylic alcohol 1 with lithium aluminum hydride does not lead to the expected vinyl allenic alcohol 2 (3,99). However, addition of solid iodine to the intermediate aluminate does effect the desired elimination to give 2. This result was unexpected because addition of a solution of iodine in THF lends to a vinyl iodide (3). This sequence provides a general synthesis of (hydroxymcthyl)allcncs (second example). 

The coupling of propargyl alcohols with Grignard reagents in the presence of HMPT and Cul provides a useful synthesis of allenes in yields of 60-95%. 

Of particular interest is that reaction of optically active propargyl alcohols results in optically active allenes. An example is the synthesis of both enantiomers of 5 from (S)-or (R)-3. 

Synthesis of Chiral Nonracemic AUenes. Enantio-merically enriched chiral allenes can be prepared by derivatization of a racemic propargyl alcohol with (R)-NEI, followed by chromatographic separation of the resulting diastereomers and reaction of the purified diastereomers with lithium dialkylcuprates at —78 °C, as shown in eq 2. ... 

Allenyltins are conveniently prepared from the terminal alkynes which have a suitable leaving group. An example is shown in equation 19. A two-step one-pot procedure for the synthesis of allene by hydrostannation of alkynes has recently been reported starting from propargylic alcohols a hydrostannation and subsequent deoxystannylation generates the allenes 6 as shown in equation 20. A chiral version of this procedure has also been described in the paper. 

A new synthesis of piperine (15) and isochavicine (16) has been reported. It is shown in outline in Scheme 3, the key step being the thermal condensation of a propargylic alcohol with an acetal to yield an allene (steps ii and iii). The mixture was separated by preparative t.l.c."... 

A general, high-yielding route to terminal allenic amides (160) is the reaction between propargylic alcohols and diethylformamide acetals,while some success has been achieved in the synthesis of potentially insecticidal 2,4,5-trienamides (161) by a Wadsworth-Emmons type reaction between substituted buta-2,3-dienals and carbamoylmethylphosphonates. ... 

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