A-allenyl

Reaction of a-allenyl alcohol 147 with methanesulfonyl chloride and triethylamine in toluene at 190 °C, in a sealed tube, led to the tricyclic dihydropyrrolizin-4-one 149 in 35% yield. This transformation involves a domino mesylation/ intramolecular Diels-Alder cycloaddition via diene 148 (Scheme 29) <2002CC1472>. 

Tius and co-workers investigated a number of cationic cyclopentannelations of allenyl ethers [113] and found that 1-lithio-l-alkoxyallenes 180 react with a,/3-unsatu-rated carbonyl compounds 181 leading to highly functionalized cyclopentenones 182 (Scheme 8.44). The primary products are a-allenyl ketones 183, which form pentadienyl cations 184 by protonation. The latter undergo a thermally allowed 4jt-conrotatory ring closure to intermediates 185, which with elimination of R1 finally lead to the expected products 182 (Scheme 8.45). 

Some observed side-products (87 or 90) can be suppressed, for example by the use of modified catalysts (Pd(dba)2/4—6equiv. PPh3 instead of Pd(PPh3)4 in Eq. 14.10). Additional model studies demonstrated the utility of a-allenyl acetates and phosphates by the successful synthesis of pheromone (R)-94 [47] and of enzyme inhibitor 95 [48] (Scheme 14.20). 

Scheme 16.96 Cross-coupling reaction of an a-allenyl alcohol and phenylacetylene.

The reaction of an allene with an aryl- or vinylpalladium(II) species is a widely used way of forming a Jt-allyl complex. Subsequent nucleophilic attack on this intermediate gives the product and palladium(O) (Scheme 17.1). Oxidative addition of palladium ) to an aryl or vinyl halide closes the catalytic cycle that does not involve an overall oxidation. a-Allenyl acids 27, however, react with palladium(II) instead of with palladium(O) to afford cr-vinylpalladium(II) intermediates 28 (Scheme 17.12). These cr-complexes than react with either an allenyl ketone [11] or with another alle-nyl acid [12] to form 4-(3 -furanyl)butenolides 30 or -dibutenolides 32, respectively. 

Scheme 17.12 Cyclization-dimerization of a-allenyl acids and ketones.

Scheme 17.13 Alternative mechanism for the cyclization—dimerization reaction of an a-allenyl acid and a ketone.

The corresponding reactions of allenyl alcohols proceed mostly via the spirodioxide intermediate (Scheme 17.21) [20], Whereas a-allenyl alcohol 61 gives the corresponding tetrahydrofuranone 62 in only moderate yield, /l-allenyl alcohols 63 react much cleaner to afford tetrahydropyranones 64 in good yield. In the case of y- and d-allenyl alcohols 65 the spirodioxide intermediate is attacked intramolecularly on the carbon atom closer to the alcohol moiety, so that tetrahydrofurans and tetrahy-dropyrans 66 are formed. 

In a similar way, propargyl esters of A -benzoyl-a-amino acids have been converted into a-allenyl-a-amino acid esters 113 by cyclization to oxazoles 111 followed by Claisen rearrangement to the 4-allenyl-2-phenyl-5(4//)-oxazolones 112. Oxazolone ring opening with methanol then afforded 113 (Scheme 7.32). ... 

Fur 4-substituierte 4-AlIenyI-5-oxo-2-phenyI-4,5-dihydro-1,3-Oxazole konnte gezeigt wer-den3-6, daB diese unter bestiminten Bedingungen in die freien a-Allenyl-aminosauren iibergefiihrt werden konnen ... 

A-Allyl compounds can be rearranged to A-propenyl derivatives. Similarly, A-propargyl compounds (969) give A-allenyl derivatives (970). 

Allenylation of acetals TiCl4 catalyzes a reaction of propargylsilanes with acetals to provide a-allenyl ethers in good yield. 

Even if the SMS reaction typically involves allylsilanes, carbonyls and alcohols (or silyl ethers), some transformations can be considered as belonging to the same family. For example, in 2001, Yokozawa et al. described [43] a three-component reaction between aldehydes 6, alkoxysilanes 38 and propargylsilane 88 (instead of allylsilane). Tritylperchlorate was used as the catalyst and a-allenyl ethers 89 were... 

Rearrangement of aryl propargyl ethers.1 Claisen rearrangement of the naphthyl propargyl ether 1 at 215° results in the benzopyran 2 as the only isolable product (40% yield). Addition of 1 equiv. of CsF results in the benzofuran 3 as the major product, presumably formed via an a-allenyl ketone (a). Related salts such as KF, RbF, or BaF2 are completely ineffective. This modified Claisen rearrangement provides a route to o-hydroxy aldehydes such as 4 from a phenol. 

Allyltriorganosilanes react with activated C-N double bonds such as iminium salts and Lewis acid-coordinated imines at the y-position to give homoallylamines.14,118 For example, in the presence of BF3, iV-acylimines generated in situ by the reaction of aldehydes or acetals with carbamates are efficiently allylated with allyltrimethylsilanes (Equation (26)).119,119a,12° The use of homochiral crotylsilanes such as 20 leads to highly diastereo- and enantioselective synthesis of homoallylamines (Equation (27)). a Allenylation of the iV-acylimines can be performed with propargylsi lanes. 

In 1986 Inanaga et al. found that allylic or propargylic acetates could be reduced in THF by diiodosamarium in the presence of a catalytic amount of a Pd(0) complex and 1 equiv of 2-propanol [160,161]. Some examples are indicated in Scheme 60. The mechanism of the reaction likely involves jt-allyl (or a-allenyl) palladium intermediates which are reduced first to radicals and then to carban-ions (see Scheme 60). A final protonation by the alcohol generates the products. 

A number of reports of the 1,4-cycloaddition of phosphites to conjugated dienes and ajS-unsaturated ketones have appeared (see also Chapter 2). a-Allenyl ketones undergo the expected cycloaddition of phosphite or phosphinite to give the phosphoranes (45), and for the chloroketone (46) cycloaddition is preferred to the alternative Perkow reaction. The adduct (47) undergoes exothermic acidic hydrolysis to give a mixture of vinylphosphonates. 

Acylation. Some noteworthy examples of acylation are the formation of 5-chloro-a-allenyl ketones, phenyl esters of P,y-unsaturated acids, and substituted cyclopentenones. ... 

The propargyl enol ethers (362) and (365) rearrange in boiling toluene to give the a-allenyl ketones (363) and (366), respectively. The allenyl ketones readily isomerize in pyridine to give the conjugated dienones (364) and (367). ... 

Although the products are not natural, the synthesis of indole derivatives shown in Scheme 37 is noteworthyJ " f Interestingly, a-allenylation and /3-propargylation with concomitant a-ethylation have been observed. The ethyl gronp is derived from EtaB used as a boron reagent. 

Acetylenic ketones are viable substrates as well for gold-catalyzed cyclizations to furans. Whereas alk-3-yn-l-ones readily cyclize to substituted furans in the presence of gold(I) or gold(III) chloride (possibly via isomerization to a-allenyl ketones see Section 5.2), the corresponding transformation of alk-4-yn-l-ones takes place in the presence of the cationic gold catalyst generated in situ from PhsPAuCl and AgOTf in toluene (Scheme 4-83). The cyclization is accelerated by... 

The cycloisomerization of a-allenyl ketones to the corresponding substituted furans was the first example of a gold-catalyzed addition of an oxygen nucleophile to an allene (Scheme 4-86). Traditionally, silver or palladium catalysts were employed for cyclizations of this type advantages of gold catalysis incluiie shorter reaction times, milder conditions, an or lower catalyst loadings. Variable amounts of... 

Kirsch has reported the conversion of propargyl vinyl ethers to form pyrroles via a multi-step transformation involving the silver-catalyzed propargyl Claisen rearrangement to form the a-allenyl P-keto ester, condensation with a primary aryl amine. 

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