Allenes electrocyclization

Reaction of lithiated allene with methoxymethyl isothiocyanate afforded 107, after trapping with methyl iodide. The newly formed 107 isomerizes under mild conditions to triene 108. This compound is ideally setup to experience an electrocyclization to dihydropyridine 109. Heating in the presence of acid facilitates aromatization of 109 to pyridines 110. 

The two-step process, depicted by path b, involves initial addition of the carbene carbon to an adjacent it bond to form bicyclo[4.1,0]hepta-2,4,6-triene (2a). This process has precedent in the analogous rearrangement of vinylcar-bene to cyclopropene (Scheme 6),lc18 and is supported by Gaspar s work on 1-cyclohexenylcarbene.17 In the second step of the mechanism in Scheme 5, subsequent six-electron electrocyclic ring opening of 2a yields the cyclic allene 3a. 

This chapter focuses on cycloaddition reactions in which at least two new cr-bonds are formed between allene derivatives and other unsaturated organic molecules. Intramole-cular cycloaddition reactions are also described. The reactions are categorized according to assembly modes, such as [m + n]-cycloaddition, where the variables m and n simply denote the number of atoms that each component contributes to the ring construction. Some electrocyclic reactions of allene derivatives are also included. 

A new benzannulation methodology was developed in order to overcome the limitations of electrocyclic ring closure of divinylindoles. The cyclization is achieved via an allene-mediated electrocyclic reaction of 2,3-difunctionalized indoles. This method is more efficient for the synthesis of highly substituted 2-methyl carbazole alkaloids (559). The 3-alkenyl-2-propargylindole 557, a precursor for the allene intermediate, was prepared from 2-formylindole over several steps using simple functional group transformations (536,537) (Scheme 5.20). 

Hibino et al. reported a formal synthesis of murrayaquinone A (107) starting from 2-chloro-3-formylindole (891) by an allene-mediated electrocyclic reaction involving the indole 2,3-bond. The 4-hydroxy-3-methylcarbazole (858), a known precursor for murrayaquinone A (107), and required for this formal synthesis was obtained in seven steps, and 26% overall yield, starting from the 2-chloro-3-formylindole (891) (636,637) (Scheme 5.114). 

Hibino et al. reported the total synthesis of carbazomycin G (269) by the regioselective addition of methyllithium onto 3-methoxy-2-methylcarbazole-l,4-quinone (941) (653). The required immediate precursor of carbazomycin G, carbazole-l,4-quinone 941, was obtained from 3-(2-methoxyethenyl)-N-(phenylsul-fonyDindole (986). The benzannulation involves an allene-mediated electrocyclic reaction of a 67t-electron system generated from the 2-propargylindole 989, which was derived from the 3-vinylindole 986 in three steps. 

Furostifoline itself has also been synthesized from 3-propargylindole using a novel allene-mediated electrocyclic reaction to produce the central core 166 in the presence of potassium /i rt-butoxide at elevated temperatures in good yield (Equation 82) <2001CPB881>. 

Intramolecular [2n + 27r] cycloadditions leading to cyclobutanes formally belong to valence isomerizations. Alkene/alkcne, ketene/alkene, and allene/alkene cycloadditions have received detailed attention. These rearrangements provide powerful methods for the synthetic arsenal. An example is the facile synthesis of bieyelo[3.2.0]heptenones 3 by intramolecular ketene/alkene cycloaddition after electrocyclic ring opening of cyclobutenoncs l.89... 

Not all electrocyclic reactions are stereoselective. It turns out that none of the three of the possible interconversions between triplet cyclopropylidene and allene should show SS, according to an analysis given by Borden (1967). 

The same palladium-catalyzed domino cyclization-anion capture sequence involving carbopalladation of allene (69), which was employed by Grigg et al. for the synthesis of diene 70 (Scheme 10) can yield 1,3,5-hexatriene 194, when starting from 2-bromo-l-ene-6-yne 193 instead of the aryl iodide 68 (Scheme 32) [52], Under the conditions of its formation, 194 immediately undergoes thermal 67r-electrocyclization to give the bicyclic product 195. 

The participation of an allenic sp center in an electrocyclic reaction was earlier demonstrated in the pyrolysis (490 C) of the pentadienylallene (339) to give o-xylere (342) together with alkyne (343). The high temperature requhad is primarily necessary to effect isomerization of the double bond to the (Z) configuration as in (340), because both the 6e electiocyclization of (340) to (341) as well as the aromat-izatitm of the latter to (342) possess lower activation barriers. That the electrocyclization of dienyl-... 

A concerted electrocyclic reaction such as that in VIII would give rise to an allene IX whose 1,3-hydrogen transfer would yield the final compound III. However, such an electrocyclic reaction does not, as yet, have precedent, and therefore may not be the most probable route. In addition, a nonconcerted electron reorganization would also furnish allene IX via a stable allylic carbocation (X). 

An unusually efficient example of an electron transfer catalyzed electrocyclic reaction proceeding via anion radicals has also been established [124]. The conversion of the bis(allene) shown in Scheme 78 to the corresponding cyclobutene derivative... 

Now we can transfer a proton from nitrogen to the middle of the allene. This is formally a i shift and is, of course, allowed but it may be the movement of a proton as nitrogen is involved gives a diene that can twist round for a six-electron electrocyclic reaction. This would no do disrotatory but we can t tell as there is no stereochemistry. 

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