Stereoselective Preparation of Six Diastereomeric Quatercyclopropanes from Bicyclopropylidene and Some Derivatives

Diastereomeric meso ‐ and d , l ‐bis(bicyclopropylidenyl) ( 5 ) were obtained upon oxidation with oxygen of a higher‐order cuprate generated from lithiobicyclopropylidene ( 4 ) in 50 and 31 % yield, respectively. Their perdeuterated analogues meso ‐[D 14 ]‐ and d , l ‐[D 14 ]‐ 5 were obtained along the same route from perdeuterated bicyclopropylidene [D 8 ]‐ 3 (synthesized in six steps in 7.4 % overall yield from [D 8 ]‐THF) in 20.5 % yield each. Dehalogenative coupling of 1,1‐dibromo‐2‐cyclopropylcyclopropane ( 6 ) gave a mixture of all possible stereoisomers of 1,5‐dicyclopropylbicyclopropylidene 16 in 69 % yield, from which ( Z )‐ cis ‐ 16 was separated by preparative gas chromatography (26 % yield). The crystal structure of meso ‐ 5 looks like a superposition of the crystal structures of two outer bicyclopropylidene units ( 3 ) and one inner s‐ trans ‐bicyclopropyl unit, whereas the two outer cyclopropyl moieties adopt a gauche orientation with respect to the cyclopropane rings at the inner bicyclopropylidene units in ( Z )‐ cis ‐ 16 . Birch reduction with lithium in liquid ammonia of meso ‐ 5 and d , l ‐ 5 gave two pairs of diastereomeric quatercyclopropanes trans , trans ‐( R *, S *, R *, S *)‐ 17 / cis , trans ‐( R *, S *, R *, R *)‐ 18 and trans , trans ‐( R *, S *, S *, R *)‐ 19 / cis , trans ‐( R *, S *, S *, S *)‐ 20 in 97 and 76 % yield, respectively, in a ratio 9:1 for every pair. The latter diastereomer was also obtained as the sole product by Birch reduction of ( Z )‐ cis ‐ 16 in 96 % yield. Under the same conditions, tetradecadeuterio analogues trans , trans ‐[D 14 ]‐( R *, S *, R *, S *)‐ 17 / cis , trans ‐[D 14 ]‐( R *, S *, R *, R *)‐ 18 (8:1) and trans , trans ‐[D 14 ]‐( R *, S *, S *, R *)‐ 19 / cis , trans ‐[D 14 ]‐( R *, S *, S *, S *)‐ 20 (12:1) were prepared from meso ‐[D 14 ]‐ 5 and d , l ‐[D 14 ]‐ 5 in 37 and 63 % yield, respectively. Reduction of meso ‐ 5 with diimine gave the cis , cis ‐quatercyclopropane ( S *, S *, R *, R *)‐ 21 as the main product (58 % yield) along with the cis , trans ‐diastereomer ( S *, S *, R *, S *)‐ 18 (29 % yield). Thus, five of the six possible diastereomeric quatercyclopropanes were obtained from meso ‐ 5 , d , l ‐ 5 , and ( Z )‐ cis ‐ 16 . The X‐ray crystal structure analyses of trans , trans ‐( R *, S *, R *, S *)‐ 17 and cis , cis ‐( S *, S *, R *, R *)‐ 21 revealed for the both an unusual conformation in which the central bicyclopropyl unit adopts an s‐ trans ‐(antiperiplanar) orientation with ϕ =180.0°, and the two terminal bicyclopropyl moieties adopt a synclinal conformation with ϕ =49.8 and 72.0°, respectively. In solution the vicinal coupling constants 〈 3 J H,H 〉 in trans , trans ‐( R *, S *, R *, S *)‐[D 14 ]‐ 17 , trans , trans ‐( R *, S *, S *, R *)‐[D 14 ]‐ 19 , trans , cis ‐( R *, S *, R *, R *)‐[D 14 ]‐ 18 and trans , cis ‐( R *, S *, S *, S *)‐[D 14 ]‐ 20 were found to be 4.1, 4.7, 5.9 and 5.9 Hz, respectively. This indicates a predominance of the all‐ gauche conformer in ( R *, S *, R *, S *)‐ 17 and a decreasing fraction of it in this sequence of the other diastereomers.