Spiro -2,4 -diones formation

Homoenolate Reactivity The ability to generate homoenolates from enals and its application to the preparation of y-butyrolactones 30, through reaction with an aldehyde or aryl trifluoromethyl ketone, was reported independently by Glorius [8], and Bode and Burstein [9] (Scheme 12.4). A sterically demanding NHC catalyst is required to promote reactivity at the d terminus and to prevent competitive benzoin dimerisation. Nair and co-workers have reported a similar spiro-y-lactone formation reaction using cyclic 1,2-diones, including cyclohexane-1,2-dione and substituted isatin derivatives [10]. 

The proposed formation of 2,5-benzothiazocine-l,6-diones (114 R = Pr) from the reaction of phthaloyl chloride (112) and amidino thioamides (113 R = Pr, Ar = 4-O2NC6H4, 4-MeOC6H4) in pyridine has been disproved. Instead, supported by an X-ray structure, the products have been shown to be spiro[4,4]lactones (116 R = Pr, Ar = 4-O2NC6H4, 4-MeOCeH4). The proposed mechanism of formation of... 

Intramolecular [2 + 2] photocycloadditions have been used successfully for the syntheses of (polycyclic) cage compounds. Recent examples of such reactions are the formation of pentacyclo[5.4.0.02 6.031°.05 9]undecane-8,ll-dione-4-spiro-l-cyclopentane,55 hexacy-clo[7.4.2.01,9.03,7.04 14.061s]pentadeca-10,12-diene-2,8-dione-5-spiro-l -cyclopropane,56 pen-tacyclo[5.4.0.02 6.031°.04 8]undecane57 and pentacyclo[6.4.0.02 7.03 12.06 9Jdodeca-4,10-diene.58 Irradiation of benzoquinone tetrahydropentalene adducts afford cage compounds which are easily converted to angular tetraquinanes.59... 

Another interesting synthesis of spiro[2.3]hexan-4-one (16) comprised of dipolar attack of 4-phenyl-l,2-dihydro-4//-l,2,4-triazole-3,5-dione on bicyclopropylidcnc to give a zwitterion 17, which underwent ring enlargement to 18 before ring closure to diaziridine 19 took place. Accordingly, in the presence of water, 18 was trapped with formation of spiro[2.3]hexan-4-one (16) which was also obtained from 19 by saponification.118... 

Initial thermal sigmatropic 1,5-carbon shifts are believed to account for the product formation from spiro[2.4]hepta-4,6-diene (l) 200 and spiro[cyclopropyl-l, 2 -2 //-indene] (6),201 obtained by photodecarbonylation of dione 5. In the first case, a 1,5-hydrogen shift and an electrocyclic ring opening completes the formation of 3,4-dimethylenecyclopent-l-ene (4),200 and in the second case, the initially formed benzoannulated bicyclo[3.2.0]hepta-l,3-diene 7 dimerizes to 8 as mixture of two stereoisomers.201... 

Figure 37. (a) Formation of the ds,trans-diol of (-)-spiro[4.4]nonane-l,6-dione. (b) Observed CD of the bis-p-dimethylaminobenzoate of the cis,trans-diol. (c) Exciton chirality. 

The reaction of 3-substituted 1,2-diphenylcyclopropenes with 4-phenyl-4//-l,2,4-triazole-3,5-dione gave urazoles, possibly via the formation of a spiro[2-azabicyclo[1.1.0]butane-2,T-l,2,4-triazole] and subsequent ring opening. 

Spiro[3,4]octa-l,5,7-triene (249) has been prepared by modification of the carboxylic acid (250). The triene was characterized chemically by reduction to the saturated hydrocarbon and by its Diels-Alder reaction with JV-phenyltriazoline-3,5-dione. Any spiroconjugative interaction in the triene is not shown on its u.v. spectrum, which has at 262 nm, the same as observed for the diene (251). The triene is unstable, rearranging at — 5°C (t = 90 min) to 6-vinylfulvene and a dihydropentalene derivative. The ring-opening of (249) to 6-vinylfulvene is unusually rapid for a 3,3-disubstituted cyclobutene derivative, and other similarly strained compounds do not show this reactivity. The ring-opening may well proceed via a biradical intermediate with stabilization of the zwitterionic form of the biradical by formation of the aromatic cyclopentadienyl anion. 

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