The stereochemistry of the Diels-Alder reaction of 1,2-dichloro-3,3-ditluorocyclopropene (Sa) to 1,4-diphenyl-1,3-butadiene (6) and 1,3-diphenylisobenzofuran (7) was unambiguously established by X-ray structure determination. In all cases known so far, tetrahalogenocyclopropenes add exo to open-chain dienes and furans. The previously reported exo-addition product (2a) of 1-bromo-2-chlorocyclopropene (5b) to 7 allows assignments of the stereochemistry of other additions of 5b to furans. exo-Addition usually predominates, but in some cases endo-adducts are also formed. This contrasts with reports in the literature that 5b adds preferentially endo to open-chain dienes.

Preparation of 1, 1‐difluorobenzocyclopropene (4) and of its 2, 5‐ and 3, 4‐dideuterio derivatives 4a and 4b is reported. Upon ionization in cold fluorosulfonic acid, 4 affords 1‐fluorobenzocyclopropenium ion (6). 1H‐ and 13C‐NMR. spectra of 4 and 6 are assigned on the basis of the data for the specifically deuteriumlabelled compounds 4a and 6a. Hydrolysis of 6a leads to 2, 5‐dideuteriobenzoic acid (7a).

The structure of 3,3,5,5-tetramethylcyclohexanone (II) is conclusively established by spectroscopy. The carbonyl stretching frequency of 1715 cm-1 (ir in carbon tetrachloride) and the position and intensity of the n- ¶*, absorption band in the uv AmaxMe0H 286 m^i; t 20) are typical for cyclohexanones. The nmr spectrum of II at 60 MHz in carbon tetrachloride shows three sharp singlets at 1.02, 1,45, and 2.10 ppm in a ratio of 12:2:4 which may be immediately assigned to the methyl, 7-methylene and a-methylene protons. The signals are sharp because the chair-chair interconversion at room temperature is so rapid on the nmr time-scale that axial and equatorial protons are indistinguishable.The spectral properties of 2-bromo-3,3,5,5-tetramethylcyclohexanone (III) are in marked contrast to those of the parent ketone. The differences stem mainly from the fact that the two interconverting chair conformations. Ilia and Ille, are not identical, but are related as dlastereomers or, more precisely since III is a racemic mixture, as enantiomeric pairs of diastereomers.

Base-induced elimination of the sulfonium salt 2i in the presence of furan affords the addition products 7 and 8, derived from 1H-cyclopropa[l]phenanthrene (1) and the isomeric cyclopropene 5a (Scheme 3). Upon oxidation, the selenide 2c yields phenanthrene-9-methanol (9), presumably via 1. No evidence for the intermediate 1 is obtained from sulfoxide pyrolysis with 2e, which leads to products formed by radical pathways (Scheme 5 ) . Reductive elimination of the disulfone 3b gives half-reduction to monosulfone 2g and complete reduction to cyclopropane 2 as well as 9methylphenanthrene (IS), but no evidence for the intermediate 1. Scheme I2 1 3 Substituted la,9b-Dihydro-lH-cyclopropa[l]phenanthrenes. -Compounds of structure 2 are in principle available via carbene addition to 9-substituted phenanthrenes [3] [6]. This approach was attempted, but abandoned when it was found impossible to effect dichlorocarbene addition to 9-bromophenanthrene under a variety of conditions. Similarly, the cyclopropan-forming procedure using diazomethane/CuBr which works for phenanthrene [7] afforded no addition product with the 9-bromo derivative. Alternatively, base-induced elimination of 1-halogenated dihydrocyclopropa[l]phenanthrenes in the presence of nucleophiles was investigated. Reaction of the dichlorocarbene adduct 4 [S] of phenanthrene with base in the presence of methanethiolate is known to afford the H E L~T I C A CHIMICA ACTA -Vol. 69 (1 986) Scheme 2 4 CI

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