Addition of dimethyl phosphonate to acetaldol gives a 3 : 7 mixture of diastereomeric dimethyl (1,3-dihydroxybutyl)phosphonates 3 and 4. The relative configurations at C-I and C-3 in these diols were established by the 13C NMR data of their 0-benzylidene derivatives. Axial preference-for the dimethoxyphosphoryl substituent at C-4 of the 1,3-dioxane ring is observed. A model of the 1,3-asymmetric induction for the Abramou reaction is proposed. Intramolecular transesterification of the phosphonates 3 and 4 in the presence of equimolar amounts of triethylamine affords diastereomeric 3-hydroxy-2-methoxy-5-methyl-1,2-oxaphospholan-2-ones. 1,2-Oxaphospholane ring opening reactions of the title compounds were studied by 31P NMR.
Stereochemie
der 1,2-Oxaphospholane, VIII '). -Synthese, Stereochemie und Ringoffnungsreaktionen diastereoisomerer 3-Hydroxy-2-methoxy-5-methyl-1,2oxaphospholan-Zone Addition von Dimethylphosphonat an Acetaldol fiihrt zu einem 3 : 7-Gemisch von diastereomeren Dimethyl-(l,3-dihydroxybutyl)phosphonaten 3 und 4. Die relativen Konfigurationen an C-1 und C-3 dieser Diole wurden '3C-NMR-spektroskopisch anhand von deren 0-Benzylidenderivaten bestimmt. Die Dimethoxyphosphorylgruppe an C-4 des 1,3-Dioxanrings nimmt bevorzugt die axiale Position ein. Es wird ein Modell fur die 1,3-asymmetrischen Induktion bei der Abramou-Reaktion vorgeschlagen. Intramolekulare Umesterung der Phosphonate 3 und 4 in Anwesenheit von aquimolaren Mengen von Triethylamin liefert die diastereomeren 3-Hydroxy-2-methoxy-5-methyl-1,2-oxaphospholan-2-one. Ringoffnungsreaktionen der im Titel genannten 1,2-Oxaphospholane wurden mittels 3'P-NMR untersucht.
Recently, we and others have shown that the addition of water to phosphites containing a C = 0 or C = N function at the y-carbon of the alkyl substituent, is an efficient method for 1,2-oxaphospholane ring closure2). This reaction also proved to be useful in the synthesis of analogues of tetrofuranosides having phosphorus in the anomeric p~sition',~).
The alternative way to this ring system involves intramolecular transesterification of (y-hydroxyalkyl)phosphonates, and several examples of this transfor-