Synthesis of endo‐ and exo‐1,3‐dimethyl‐2,9‐dioxabicyclo[3.3.1]nonane
The synthesis of a host‐specific substance in norway spruce infested by Trypodendron lineatum OLIV. is described (cf. scheme 1 and 2). Alkylation of the acetyl‐acetone di‐anion (II) with 3‐methyl‐3‐buten‐1‐yl‐bromide (I) followed by sodium boro‐hydride reduction yields erythro‐ and threo‐8‐methyl‐8‐nonen‐2,4‐diol (IV and V) which are separated by chromatography. Their configurations were established by converting them under equilibrium conditions into one (VI) or two (VII and VIII) benzal derivatives. Oxidative cleavage with ozone of the terminal double bond in the erythro diol IV produces a dihydroxy ketone IX which spontaneously cyclizes to endo‐1,3‐dimethyl‐2,9‐dioxa‐bicyclo[3.3.1]nonane (X). The threo diol V is converted by the same reaction sequence exclusively into exo‐1,3‐dimethyl‐2,9‐dioxa‐bicyclo‐[3.3.1]nonane (XII). Comparison of the NMR. data of the two acetals X and XII with that of the natural product establishes the endo configuration of the latter. A second, more convenient, synthesis of a mixture of the acetals X and XII starting from the bromo‐acetal XIII is also reported.