NOVELTY - Preparation (M1) of 2-(4-(4-chlorophenyl)cyclohex-1-enyl)-3,4-dihydronaphthalen-1(2H)-one (V) involves (i) reacting 2-(4-(4-chlorophenyl)-1-hydroxycyclohexyl)-3,4-dihydronaphthalen-1(2H)-one (IV) with trifluoroacetic anhydride in presence of base in organic solvent to yield 4-(4-chlorophenyl)-1-(1-oxo-1,2,3,4-tetrahydronaphthalen-2-yl)cyclohexyl-2,2,2-trifluoroacetate (XIa); and (ii) eliminating trifluoroacetyl functionality of compound (XIa) in organic solvent and in presence of organic base to give compound (V).    USE - For preparing 2-(4-(4-chlorophenyl)cyclohex-1-enyl)-3,4-dihydronaphthalen-1(2H)-one used as intermediate for preparation of atovaquone (claimed) useful for treatment of Pneumocystis carinii pneumonia, malaria, toxoplasmosis and carcinoma or fibrosarcoma; and for inhibition of mitochondrial electron transport in cytochrome of the parasite.    ADVANTAGE - The method is cost effective, green, and eco-friendly process.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) preparation of compound (XIa) which comprises step (i) per se;    (2) compound (XIa); and    (3) conversion of compound (V) into atovaquone (I) involving hydrogenation of 2-(4-(4-chlorophenyl)cyclohex-1-enyl)-3,4-dihydronaphthalen-1(2H)-one (V) with platinum oxide to obtain cis/trans mixture of 2-(4-(4-chlorophenyl)cyclohexyl)-3,4-dihydronaphthalen-1(2H)-one (VI); ketone bromination of cis/trans mixture of (VI) to obtain cis/trans mixture of 2-bromo-2-(4-(4-chlorophenyl)cyclohexyl)-3,4-dihydronaphthalen-1(2H)-one (VII); elimination of cis/trans mixture of (VII) with a strong base to give cis/trans mixture of 2-(4-(4-chlorophenyl)cyclohexyl)naphthalen-1-ol (VIII); oxidizing cis/trans mixture of (VIII) to obtain cis/trans mixture of 2-(4-(4-chlorophenyl)cyclohexyl)naphthalene-1,4-dione (IX); base catalyzed epoxidation of cis/trans mixture of (IX) to cis/trans mixture of 1a-(4-(4-chlorophenyl)cyclohexyl)naphtho(2,3-b)oxirene-2,7(1aH,7aH)-dione (X) in presence of hydrogen peroxide; acid catalyzed hydrolysis of cis/trans mixture of 1a-(4-(4- chlorophenyl)cyclohexyl)naphtho(2,3-b)oxirene-2,7(laH,7aH)-dione (X) to obtain 2-(trans-4-(4'-chlorophenyl)cyclohexyl)-3-hydroxy-1,4-naphthoquinone (I).