A process for preparing eribulin, or a pharmaceutically acceptable salt thereof, said process comprising the steps of: (a) producing a compound of formula comprising steps of: (i) reacting compound of Formula (Ia) with a protecting agent in the presence of a base to obtain compound of Formula (Ib) (ii) reacting with a suitable hydroxylating agent, to form vicinal diol of Formula (Ic) (iii) protecting using suitable protecting agent to form compound of Formula (Id): (iv) reducing with suitable reducing agent to form compound of Formula (Ie): reacting with suitable halogenating agent to form compound of Formula (If): and (vi) alkynation and haloboration to form compound of Formula; wherein PG 1, PG 2 and PG 3 are independently hydrogen or taken with the oxygen atom to which they are bound are silyl ethers or arylalkyl ethers or are mesyl or tosyl, and X 1, X 2 and X 3 are leaving group, selected from halide or sulfonate; or wherein X 1 is iodo, X 2 and X 3 are bromo, PG 1 is mesyl (Ms), PG 2 and PG 3 are t-butyldimethyl silyl, producing a compound of Formula or -A from the compound of formula, wherein each of PG 4 and PG 5 are independently H; or PG4 and PG5, together with the oxygen atoms to which they are attached, form a diol protecting 5-membered heterocyclic ring, which is optionally substituted with C1-4 alkyl groups; R 1 and R 2 each independently is H, -CH 2 OR 3 or-CH 2 SO 2 Ar, or R 1 and R 2 together form =CH 2 SO 2 Ar, wherein R 3 is H or a hydroxyl protecting group; and Ar is an aryl group; and R is H, C1-6 alkyl or C1-6 haloalkyl, (b) reacting the compound of Formula or -A under suitable conditions to produce eribulin, or the pharmaceutically acceptable salt thereof. (a) producing a compound of formula comprising steps of: (i) reacting compound of Formula (Ia) with a protecting agent in the presence of a base to obtain compound of Formula (Ib) (ii) reacting with a suitable hydroxylating agent, to form vicinal diol of Formula (Ic) (iii) protecting using suitable protecting agent to form compound of Formula (Id): (iv) reducing with suitable reducing agent to form compound of Formula (Ie): reacting with suitable halogenating agent to form compound of Formula (If): and (vi) alkynation and haloboration to form compound of Formula; wherein PG 1, PG 2 and PG 3 are independently hydrogen or taken with the oxygen atom to which they are bound are silyl ethers or arylalkyl ethers or are mesyl or tosyl, and X 1, X 2 and X 3 are leaving group, selected from halide or sulfonate; or wherein X 1 is iodo, X 2 and X 3 are bromo, PG 1 is mesyl (Ms), PG 2 and PG 3 are t-butyldimethyl silyl, producing a compound of Formula or -A from the compound of formula, wherein each of PG 4 and PG 5 are independently H; or PG4 and PG5, together with the oxygen atoms to which they are attached, form a diol protecting 5-membered heterocyclic ring, which is optionally substituted with C1-4 alkyl groups; R 1 and R 2 each independently is H, -CH 2 OR 3 or-CH 2 SO 2 Ar, or R 1 and R 2 together form =CH 2 SO 2 Ar, wherein R 3 is H or a hydroxyl protecting group; and Ar is an aryl group; and R is H, C1-6 alkyl or C1-6 haloalkyl, (i) reacting compound of Formula (Ia) with a protecting agent in the presence of a base to obtain compound of Formula (Ib) (ii) reacting with a suitable hydroxylating agent, to form vicinal diol of Formula (Ic) (iii) protecting using suitable protecting agent to form compound of Formula (Id): (iv) reducing with suitable reducing agent to form compound of Formula (Ie): reacting with suitable halogenating agent to form compound of Formula (If): and (vi) alkynation and haloboration to form compound of Formula; wherein PG 1, PG 2 and PG 3 are independently hydrogen or taken with the oxygen atom to which they are bound are silyl ethers or arylalkyl ethers or are mesyl or tosyl, and X 1, X 2 and X 3 are leaving group, selected from halide or sulfonate; or wherein X 1 is iodo, X 2 and X 3 are bromo, PG 1 is mesyl (Ms), PG 2 and PG 3 are t-butyldimethyl silyl, producing a compound of Formula or -A from the compound of formula, wherein each of PG 4 and PG 5 are independently H; or PG4 and PG5, together with the oxygen atoms to which they are attached, form a diol protecting 5-membered heterocyclic ring, which is optionally substituted with C1-4 alkyl groups; R 1 and R 2 each independently is H, -CH 2 OR 3 or-CH 2 SO 2 Ar, or R 1 and R 2 together form =CH 2 SO 2 Ar, wherein R 3 is H or a hydroxyl protecting group; and Ar is an aryl group; and R is H, C1-6 alkyl or C1-6 haloalkyl, wherein PG 1, PG 2 and PG 3 are independently hydrogen or taken with the oxygen atom to which they are bound are silyl ethers or arylalkyl ethers or are mesyl or tosyl, and X 1, X 2 and X 3 are leaving group, selected from halide or sulfonate; or wherein X 1 is iodo, X 2 and X 3 are bromo, PG 1 is mesyl (Ms), PG 2 and PG 3 are t-butyldimethyl silyl, producing a compound of Formula or -A from the compound of formula, wherein each of PG 4 and PG 5 are independently H; or PG4 and PG5, together with the oxygen atoms to which they are attached, form a diol protecting 5-membered heterocyclic ring, which is optionally substituted with C1-4 alkyl groups; R 1 and R 2 each independently is H, -CH 2 OR 3 or-CH 2 SO 2 Ar, or R 1 and R 2 together form =CH 2 SO 2 Ar, wherein R 3 is H or a hydroxyl protecting group; and Ar is an aryl group; and R is H, C1-6 alkyl or C1-6 haloalkyl, (b) reacting the compound of Formula or -A under suitable conditions to produce eribulin, or the pharmaceutically acceptable salt thereof.