The macrocyclic pentolide 1, hexolide 2, and heptolide 3 constitute ca. 80% of the oligomers formed in ca. 50% yield from enantiomerically pure 3-hydroxybutanoic acid under Yamaguchi's macrolactonization conditions. The FAB mass spectra of the MH', MNaf, and MCsf are reported (Figs. 2, 3, 5, and 6 ) . No cyclic tetramer is detected. The 'H-NMR spectra of the cyclic oligomers, of the monomer, and of the polymer (PHB) are very similar (Fig. 4 ) . Directed synthesis of the open-chain dimer and tetramer of 3-hydroxybutanoic acid and attempted cyclization do not lead to the isolation of the cyclic tetramer.

Offenkettige (2-Nitropropan, -butan, -pentan, -hexan) und cyclische (Nitrocyclopentan, -hexan) sekundare Nitroalkane (1) werden mit je einem Aquivalent n-und t-Butyllithium (THF/HMPT oder DMPU) doppelt deprotoniert. Es entstehen Losungen von Bis(1ithio-0xy)enaminen (2, Dianionderivate von Nitroolefinen, Super-enamine), und zwar aus den 2-Nitroalkanen nur diejenigen mit endstindiger Doppelbindung (2a -d). Umsetzungen rnit Benzylbromid, Benzoesaureester, aliphatischen oder aromatischen Aldehyden und einem Keton fuhren in Ausbeuten zwischen ca. 20 und 60% zu Produkten 4-11. Die aus den offenkettigen Nitroalkanen und Aldehyden erhaltenen 3-Nitropropanol-Derivate (9a -m, Tab. 1) sind ca. 1 : I-Diastereomerengernische, wahrend die cyclischen Derivate (12, 13) rnit drei neu gebildeten Chiralitatszentren stereoselektiv (60 bis iiber 95% ds) entstehen. Die Konfiguration des Hauptprodukts (12e) aus Benzaldehyd und Nitrocyclopentan wird NMRspektroskopisch zugeordnet und fuhrt zur Spezifikation lk,lk-1,2 fur die vorliegende asymmetrische Synthese. Damit zeigt die Reaktion bevorzugt denselben sterischen Verlauf der C,C-Verkniipfung wie entsprechende Aldol-Additionen, Munnich-Reaktionen oder MichuelAdditionen an Nitroolefine, rnit anschlieknder asymmetrischer Protonierung des primar gcbildeten Nitronats (s. Schema 3 sowie G und H). Regioselective Generation and Diastereoselective Reactions in the &Position of Nitro Groups of Secondary Nitroalkanes througb a,gDoubly Deprotonated Derivatives (Super-enamines)Open-chain (2-nitropropane, -butane, -pentane, -hexane) and cyclic (nitrocyclopentane, -hexane) secondary nitroalkanes (1) were doubly deprotonated with one equivalent each of n-and t-butyllithium in THF/HMPT or DMPU. Bis(1ithiooxy)enamines (2, dianion derivatives of nitroolefins, super-enamines), are generated, exclusively those with a terminal double bond (Za -d) from the 2-nitroalkanes. The dianion derivatives reacted with benzyl bromide, alkyl benzoates, aliphatic or aromatic aldehydes, and a ketone to give the products 4-11 in yields between 20 and 60%. The derivatives of 3-nitropropanol(9a-m, Table l), which were obtained from the open-chain nitroalkanes and aldehydes, were ca. 1 : 1-mixtures or diastereoisomers, whereas the cyclic derivatives (12, 13) with three newly created centers of chirality were formed stereoselectively (60 to more than 95% ds). The configuration of the main product (12e) obtained from benzaldehyde and nitrocyclopentane was assigned by NMR spectroscopy and leads to the specification lk.Ik-1,2 for this asymmetric synthesis.

Dedicated to Prof. Dr. Wilhelm Simon on the occasion of his 60th birthday (27.VII.89)The temperature and concentration dependence of the previously reported formation of oligolides from (R)-or (S)-3-hydroxybutanoic acid under Yumuguchi's macrolactonization conditions (2,4,6-trichlorobenzoyl chloride/base) was studied. While the content of hexolide 2 in the product mixture is almost invariably cu. 35%, the amounts of pentolide 1 and of the larger rings strongly depend upon the temperature employed (Fig. 1). Cyclic oligomers (5,6) are also obtained from 3-hydroxypentanoic acid. Enantiomerically pure B-butyrolactone can be used for the preparation of pento-, hexo-, and heptolide under Shunzer's macrolactonization conditions (tetraoxadistannacyclodecane 'template'). The X-ray crystal structures of the pentolide 1 and of two modifications (space groups C 2 and P 2,) of the hexolide 2 were detcrmined (Figs. 2 4 and Tables 1 and 5 ) . No close contacts between substituent atoms and atoms in the rings or between ring atoms are observed in these structures. The hexolide C 2 modification is 'just a large ring', while the crystals of the P 2, modification contain folded rings the backbones of which resemble the seam of a tennis ball. A comparison of the torsion angles in the folded hexolide ring of the P 2, modification with those in the helical poly-(R)-3-hydroxybutdnoate (PHB) suggests (Table 2) that the same interactions might be responsible for folding in the first and helix formation in the second case. Molecular modeling with force-field energy minimization of the tetrolide from four homochiral B-hydroxybutanoic acid units was undertaken, in order to find possible reasons for the fact that we failed to detect the tetrolide in the reaction mixtures. The calculated conformational energies (per monomer) for some of the tetrolide models (Figs. 7-9 and Tables 3 and 4 ) are not significantly higher than for the pentolide and hexolide crystal structures. We conclude that thermodynamic instability is an unlikely reason for the lack of tetrolide isolation. This result and failure to observe equilibration of pentolide 1 to a mixture of oligomers under the reaction conditions suggest that product distribution is governed by kinetic control.

The biopolymers PHB and PHB/PHV (mol. weight > 7·105 g·mol?1) from fermentations of certain lot numbers or PHB samples which had previously been precipitated from dichloroethane solution can be dissolved by treatment with excess lithium-di-isopropylamide in LiCl-containing tetrahydrofuran. The samples recovered from these solutions in good yields have molecular weights of 1000–5000 g·mol?1.