5.5- Dimethoxy-2-pentanone

Reaction of 5,5-dimethoxy-2-pentanone with methylmagnesium iodide followed by treatment with aqueous acid forms cyclic hemiacetal Y. Draw a stepwise mechanism that illustrates how Y is formed. 

In another approach, several ct-hydroxydimethylacetals 258, available according to Scheme la, undergo bimolecular loss of MeOH under acid catalyst, to yield the corresponding 1,4-dioxanes 259 (84JOC4581) (Eq. 42). In the case of 3,3-dimethoxy-2-pentanol (260), which is prepared from 3-pentanone, the cyclization gives dioxane 261 rather than the expected 262 (Scheme 65). The rearrangement of 260 to a mechanistically reasonable precursor of 261 is well known from the work of Creary and Rollin (77JOC4231). 

The starting material in the synthesis of 106 (157) was 3-(3,4-dimethoxy-phenyl)propanoyl chloride, which with the aid of diazomethane at — 20°C, was converted to the diazo ketone 108. Treatment of 108 in ether with 48% hydrogen bromide gave 4-bromo-l-(3,4-dimethoxyphenyl)-3-pentanone (109). The bromo ketone 109 was heated in an autoclave with 2-iminotetra-hydropyran hydrochloride in methanolic ammonia and yielded the product 106, identical to the alkaloid cypholophine, in an overall yield of 6%. 

Acetyl-3,5,6-trimethoxyphenyl)-1 -propanone, 2135 3-Butyl-2,4,6-trihydroxy-5-(l-oxopropyl)benzaldehyde, 2135 1,1 -(2-Hydroxy-4,6-dimethoxy-1,3-phenylene)bis-1 -propanone, 2114 1,1 -(6-Hydroxy-2,4-dimethoxy-1,3-phenylene)bis-1 -propanone, 2114 1 - [4-Hydroxy-3 -[2-( 1 -oxopropoxy)ethoxy ]phenyl] -1 -propanone, 1908 1 -[2,4,6-Trihydroxy-3-(1 -oxopropyl)phenyl]-1 -pentanone, 2136 l,r-(2,4,6-Trihydroxy-l,3-phenylene)bis[2-methyl-l-propanone, 2147... 

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