Pentadienylation carbonyl compounds

Allyl chlorides are readily deprotonated a to the halide by strong bases to give allyl carbenoids, which insert into organozirconocene chlorides to afford allyl- or pentadienyl-zirco-nocene chlorides (Scheme 3.23). These allylmetallic species are reactive towards carbonyl compounds, and so an efficient three-component coupling results [50]. 

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). 

As far as we are aware, these observations are the first that show that the well-known Norrish Type I reactions of p,7-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. This unusual reactivity may be due to the fact that the TiC-ir, -ir ) excited states of 53 and 55 possess sufficient energy to promote the homolytic allylic bond fission to form the stabilized pentadienyl radical 57. As a result, photodecarbony-lation competes favorably with the ODPM rearrangement. 

There has been one report of an rj1 -pentadienyl complex of a transition element, Cp2Zr(2-MeC5H6)2, prepared from Cp2ZrCl2 and 2-methyl-pentadienylpotassium (155). Cp2ZrHCl, however, give -complexes of 1,3-pentadiene and its higher homologs, when treated with pentadienyl-potassiums. The reactions of these products with alkenes, alkynes, dienes, and carbonyl compounds are synthetically important (156). 

Pentadienyl hydroxy acids 7-hydroxy-2,4-dienoic acids The dianion (2) of 1 reacts with carbonyl compounds to afford either pentadienyl hydroxy acids (3) or 7-hydroxy-2,4-dienoic acids (4), depending on the reaction conditions. Reaction... 

Pentadienyl compounds similar to -C6H7Mn(CO)3 are also prepared by the reaction of pentadiene compounds of organotin compounds with manganese carbonyl compounds as shown in eq. (14.11) [25]. 

Another way to circumvent the stereoselectivity issues that may arise from olefination or addition/elimination sequences to l,4-dien-3-ones is to switch the polarity of components and olefinate a carbonyl compound with a symmetrical nucleophilic pentadienyl anion equivalent. A seminal contribution was reported by Paul and Tchelitcheff in 1951, who combined trivinylmethane (235) and carbon dioxide to form a [3]dendralene 237 (Scheme 1.40 (a)) [190]. In this instance, the anion of trivinyl methane 236 is indeed a pentadienyl anion, but bond formation occurs with allylic transposition through a vinyl unit. 

The alkynyl group forms stable complexes with a number of r-cyclo-pentadienyl and carbonyl compounds, e.g. 

A recent application of the furan-carbonyl photocycloaddition involved the synthesis of the mycotoxin asteltoxin (147)." Scheme 16 shows the synthetic procedure that began with the photoaddition of 3,4-dimethylfuran and p-benzyloxypropanal to furnish photoaldol (148), which was epoxidized with MCPBA to afford the functionalized product (149) in 50% overall yield. Hydrolysis (THF, 3N HCl) provided the monocyclic hemiacetal which was protected as its hydrazone (150). Chelation-controlled addition of ethylmagnesium bromide to the latent a-hydroxy aldehyde (150) and acetonide formation produced compound (151), which was transformed through routine operations to aldehyde (152). Chelation-controlled addition of the lithium salt of pentadienyl sulfoxide (153) followed by double 2,3-sigma-tropic rearrangement provided (154) as a 3 1 mixture of isomers (Scheme 17). Acid-catalyzed cyclization of (154) (CSA/CH2CI2) gave the bicyclic acetal (155), which was transformed in several steps to ( )-asteltoxin (147). ... 

Piper, Cotton, and Wilkinson in 1955 noted that stabilization effects also result from combining carbonyl and/or nitrosyl groups with a 7r-cyclo-pentadienyl ring 195), Compounds of this type were prepared as follows ... 

Electrocyclic reactions are not limited to neutral polyenes. The cyclization of a pentadienyl cation to a cyclopentenyl cation offers a useful entry to five-membered carbocycUc compounds. One such reaction is the Nazarov cyclization of divinyl ketones. Protonation or Lewis acid complexation of the oxygen atom of the carbonyl group of a divinyl ketone generates a pentadienyl cation. This cation undergoes electrocyclization to give an allyl cation within a cyclopentane ring. The allyl cation can lose a proton or be trapped, for example by a nucleophile. Proton loss occurs to give the thermodynamically more stable alkene and subsequent keto-enol tautomerism leads to the typical Nazarov product, a cyclopentenone (3.220). 

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