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Abstract
Benzynes were generated selectively through loss of ortho-bromotosylate from 1,2-dibromo-3- tosylates. Thus when treated with bu tyl lithium in the presence of furan rel -(2 R ,4 S ,5 R )-4-(2',3'- dibromo-5'-methoxy-4'-toluene- p -sulfonyloxyphenyl)-2,5-d imethyl-1,3-dioxolane 21 was converted in two steps into rel -(2 R ,4 S ,5 R )-4-(1'-bromo-4'-methoxynaphthalen-2'-yl)-2,5- dimethyl-1,3-dioxolane 8 in good yield. Attempted stereoselective isomerization of dioxolane 8 with titanium(IV) chloride at low temperature led to the recovery, almost exclusively, of starting material. The debrominated analogue rel -(2 R ,4 S ,5 R )-4-(1'-methoxynaphthalen-3'-yl)-2,5- dimethyl-1,3-dioxolane 31 , on the other hand, isomerized readily to give rel -(1 R ,3 R ,4 S )- and rel - (1 S ,3 R ,4 S )-3,4-dihydro-4-hydroxy-6-met hoxy-1,3-dimethylnaphtho[1,2- c ]pyrans 32 and 34 in a ratio (~1:3) that did not vary with reaction temperature.
Highlights
Reductive cleavage of the aphid insect pigments the protoaphins-fb 1 and –sl 2 affords glucoside B 3 in each case as well as, for the former, quinone A 4 and for the latter, quinone A' 5,1 as shown in Scheme 1
Dimethyldioxolanes were found to provide the 1,3-trans-3,4-trans stereochemistry required for glucoside B since the stereochemistries at C-4 and c and 152.2 (C-5) in the dioxolanes were transferred unaltered to C-4 and C-3 respectively in the product benzo-and naphthopyrans.[6]
Having subsequently optimised the reaction conditions for this isomerization[5−7] we report on the further examination of the rearrangement of a naphthyldioxolane bearing a blocking bromine atom ortho to the dioxolane ring with a view to assembling linear naphthopyrans
Summary
Reductive cleavage of the aphid insect pigments the protoaphins-fb 1 and –sl 2 affords glucoside B 3 in each case as well as, for the former, quinone A 4 and for the latter, quinone A' 5,1 as shown in Scheme 1. Isomerization of the corresponding naphthyldioxolane lacking this bromine substituent gives an approximately 1:3 mixture of the two angular naphthopyrans and 34 in good yield and this ratio remains the same when the reaction is performed over a range of temperatures from −95 oC to −30 oC. The filtrate, containing mostly tosyl chloride, was discarded and the residue (mostly product) was further purified by dry packed chromatography (20% ethyl acetate-hexane) to give the product 13 as a light brown solid (6.32 g, 84%).
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