NOVELTY - Preparing 7-phenylacetamido-3-vinyl cephalosporanic acid-para-methoxybenzyl ester involves: dissolving 7-phenylacetamido-3-chloromethyl-3-cephem-4-carboxylic acid-para-methoxybenzyl ester in organic solvent; reacting triphenylphosphine and quaternary amine halide with solution obtained to form ((2-((4-methoxybenzyloxy)carbonyl)-8-oxo-7-(2-phenylacetamido)-5-thia-1-azabicyclo(4.2.0)oct-2-en-3-yl)methyl)triphenylphosphonium salt; adding organic solvent; treating filtrate with aqueous formaldehyde and base; adjusting pH with acid; precipitating organic layer; adding water; and isolating.    USE - For the preparation of 7-phenylacetamido-3-vinyl cephalosporanic acid-para-methoxybenzyl ester (claimed), which is useful as a key intermediate for the preparation of cephalosporins having antibacterial activity (e.g. Cefixime and Cefdinir) against a range of gram-positive and gram-negative organisms.    ADVANTAGE - The process provides a simple, economical, user-friendly and commercially viable process for the preparation of 7-phenylacetamido-3-vinyl cephalosporanic acid-para-methoxybenzyl ester (I), which is the key intermediate for the preparation of cephalosporins; by utilizing a quaternary amine halide compound. The process provides higher yield with greater purity of the Wittig product, and hence the compound (I). In the prior art, the use of alkali metal iodides such as sodium iodide in Wittig reaction caused the tendency of product to pick up color for which it required additional purification procedures; whereas the process uses substantially cheaper reagent(s) in substantially lower amounts and less organic solvent(s), and achieves better quality Wittig product that is substantially free of colored impurities forming in the reaction. The process avoids excessive use of reagents such as sodium bromide, sodium iodide, triphenylphosphine, formaldehyde, and large amounts of organic solvent(s) such as N,N-dimethylformamide and methylene chloride; which reduces the reaction bulk and facilitates scaling up of the process for an industrial production. The triphenylphosphonium halide formation and vinylation steps are carried out in an in-situ manner without isolating the compound (III), which makes the process easier. The process has effectively less number of steps that result in shortening of reaction time and lowering of labor. Thus the process is easy to implement on commercial scale. As the process avoids excess usages of reagent(s) and organic solvent(s), it promotes green chemistry i.e. is eco-friendly, and ensures a cleaner surrounding by putting less load on environment. The addition of water in step (G) is a very essential and crucial feature of the process. This results in better quality of the compound (I) along with good yield, as the addition of water during this step takes care of the inorganics or by-products formed during the reaction. Further, the by-products formed during the reaction can be reusable and recyclable, which makes the process industrially and commercially viable.    DETAILED DESCRIPTION - Preparation of 7-phenylacetamido-3-vinyl cephalosporanic acid-para-methoxybenzyl ester (I), involves:    (A) dissolving 7-phenylacetamido-3-chloromethyl-3-cephem-4-carboxylic acid-para-methoxybenzyl ester (II) in an organic solvent to get a solution;    (B) reacting triphenylphosphine and a quaternary amine compound of formula (R')4N+X-, with the solution obtained from the step (A) to obtain ((2-((4-methoxybenzyloxy)carbonyl)-8-oxo-7-(2-phenylacetamido)-5-thia-1-azabicyclo(4.2.0)oct-2-en-3-yl)methyl)triphenylphosphonium salt (III);    (C) adding an organic solvent after the completion of reaction;    (D) filtering the reaction mixture;    (E) treating the filtrate with aqueous formaldehyde and a base;    (F) adjusting the pH of reaction mixture with acid;    (G) separating and concentrating the organic layer;    (H) precipitating the reaction mixture by using an alcoholic solvent;    (I) adding water to the reaction mixture; and    (J) isolating the compound (I).    R'=alkyl, aryl, arylalkyl (all optionally substituted), or H;    X=halo selected from Br and I.