Secondary propargylic

The isomerization of propargylic alcohols to a,/3-enones was also developed by using an iridium catalyst. A variety of secondary propargylic alcohols can be isomerized to the corresponding a,/3-unsaturated ketones in the presence of 1 mol% IrHs(Pr 3P)2 (Scheme 54).93... 

The steric course of this two-step process was examined with several chiral secondary propargylic phosphates (Eq. 9.35) [40], The derived propargylic stannanes were found to be formed with net inversion of configuration. Based on previous evidence that the initial formation of the allenyltitanium intermediate occurs with inversion, it can be deduced that stannylation proceeds by a syn pathway. This surprising result was attributed to coordination between the chlorine substituent of the Bu3SnCl and the electropositive titanium center (Scheme 9.11). 

Primary propargylic formates decarboxylate in the presence of Pd(acac)2 and Bu3P at room temperature to give mainly allenic products (Eq. 9.115) [91]. Initial formation of a propargylic palladium complex, which rearranges to the more stable allenylpalladium species, accounts for this transformation. Under similar conditions, a terminal allenyl formate afforded a 99 1 mixture of allene and acetylene product (Eq. 9.116) [91]. However, a mixture of enyne elimination products was formed when a secondary propargylic carbonate was treated with a palladium catalyst (Eq. 9.117). 

Secondary propargylic amines 284 (R = Pr, t-Bu, C5H11, C6H13 or C7H15) rearrange to the enamines 285 and thence to the imines 286 in the presence of potassium r-butoxide313. 

Secondary propargyl alcohols (17 R2 = H) require rather stronger acid and a longer reaction time, and the resulting bromoallenes may contain up to 5 per cent of the 3-bromoacetylenes, which may, however, be removed by fractional distillation. 

In contrast with the catalytic system based on RuCl(rf-C9H7)(PPh3)2 in micellar solutions [32], the reaction of secondary propargylic alcohols in 2-propanol/ H20 at 100 °C in the presence of 5 mol % of RuCl(Cp)(PMe3)2 leads to conjugated enals with E stereoselectivity (Eq. 12) [88]. 

The electrophilic propargylation at the C-2-position of furans with propargylic alcohols can be effected by using 5 mol% of the cationic methanethiolate diruthenium complex 41 as a catalyst (Equation 29). Substrates are limited to 1-phenyl-substituted secondary propargylic alcohols <2003AGE1495>. 

The acetals 2 undergo a similar coupling with l-trimethylsilylalkynes, also with high diastereoselectivity, to furnish secondary propargylic alcohols after elimination of the chiral auxiliary (equation 111). ... 

It has been found that allenyltitaniums (134), prepared in situ by the reaction of optically active secondary propargyl phosphates (135) with a divalent titanium reagent, react readily with alkenylidenemalonates with excellent regio- and dia-stereoselectivity to afford the Michael addition products (136) with high optical purity (Scheme 33). ... 

The same results are observed by Tadema et al. [83)]. The anr/-1,3-substitution route is the normal stereochemical pathway for secondary propargylic sulfinates, whereas the syn-route is preferred if the ester is derived from tertiary propargylic alcohols. 

In the laboratory of D. Tanner, a novel method was developed for the stereoselective synthesis of ( )-tributylstannyl-a,P-unsaturated ketones in two steps from secondary propargylic alcohols. The first step was the highly regio- and stereoselective Pd-catalyzed hydrostannylation of the triple bond followed by a mild Ley oxidation. This method was utilized for the construction of a key intermediate for the total synthesis of zoanthamine. 

H. Stark and co-workers prepared novel histamine Hs-receptor antagonists with carbonyl-substituted 4-[(3-phenoxy)propyl]-1/-/-imidazole structures. The Meyer-Schuster rearrangement was used for the synthesis of one of the compounds. The p-hydroxybenzaldehyde derivative was reacted with ethynylmagnesium bromide to afford a secondary propargylic alcohol. Upon hydrolysis with 2N HCI in a refluxing ethanol/acetone mixture, the corresponding p-hydroxy cinnamaldehyde was obtained. 

Optically active allylstannanes, R3SnC HR (CH=CHR"), are formed from active secondary propargyl phosphates from the sequence of reactions shown in equation 9-4,8 using the Ti(OPr1)4/Pr1MgBr reagent.9 Presumably the titanium/tin transmetallation involves an Sn2 process. 

A changeover of regioselectivity was observed by Mikami et al. in the reduction of secondary propargylic phosphates by the SmI2/Pd(0)/proton source system [169]. ferf-Butanol and dimethyl (R,R)-tartrate gave allene and acetylene, respectively (Scheme 61). This process was modified in an asymmetric synthesis of chiral allenes [170]. 

Chiral 1,3-dialkylallenes. These aUenes can be obtained by reaction of lithium dialkylcuprates with one of the diastereomeric carbamates obtained from racemic secondary propargylic alcohols and the reagent (1). An example is the synthesis of chiral pheromone of the male bean weevil, (R)-( —)-6, as outlined in scheme (I). ... 

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