Stereoselective Preparation of Six Diastereomeric Quatercyclopropanes from Bicyclopropylidene and Some Derivatives

AbstractDiastereomeric 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‐[D14]‐ and d,l‐[D14]‐5 were obtained along the same route from perdeuterated bicyclopropylidene [D8]‐3 (synthesized in six steps in 7.4 % overall yield from [D8]‐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‐[D14]‐(R*,S*,R*,S*)‐17/cis,trans‐[D14]‐(R*, S*,R*,R*)‐18 (8:1) and trans,trans‐[D14]‐(R*,S*,S*,R*)‐19/cis,trans‐[D14]‐(R*,S*,S*,S*)‐20 (12:1) were prepared from meso‐[D14]‐5 and d,l‐[D14]‐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 〈3JH,H〉 in trans,trans‐(R*,S*,R*,S*)‐[D14]‐17, trans,trans‐(R*,S*,S*,R*)‐[D14]‐19, trans,cis‐(R*,S*,R*,R*)‐[D14]‐18 and trans,cis‐(R*,S*,S*,S*)‐[D14]‐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.