A method of preparing an intermediate in the synthesis of eribulin, said method comprising performing a macrocyclization reaction on a non-macrocyclic intermediate, said macrocyclization reaction producing said intermediate in the synthesis of eribulin by forming a C.15-C.16 bond in the structure of eribulin, wherein said performing said macrocyclization reaction comprises contacting said non-macrocyclic intermediate with an olefin metathesis catalyst, wherein said non-macrocyclic intermediate is a compound of formula or a salt thereof: wherein R 1 is H or -CH 2 X 1 CH 2 CH=CH 2, wherein X 1 is O, -C(R 9) 2 -, or NP 5, and wherein each R 9 is independently H or -COOR 10, P 5 is an N -protecting group, and R 10 is C 1-6 alkyl; R 2 is H or a hydroxyl protecting group, R 3 is C 1-6 alkyl ether, and R 4 is H; R 2 is H or a hydroxyl protecting group, and R 3 and R 4 combine to form a double bond; or R 2 and R 3 combine to form a bond, and R 4 is H; R 5 is H or a hydroxyl protecting group, and R 6 and R 7 combine to form a double bond; or R 5 and R 6 combine to form a bond, and R 7 is H; R 8 is H, and P 4 is H or a hydroxyl protecting group; or R 8 and P 4 combine to form a double bond; each P 3 is independently H or a hydroxyl protecting group; and X is O, and each of P 1 and P 2 is independently H or a hydroxyl protecting group, or P 1 and P 2, together with the atoms to which each is attached, combine to form a cyclic protected diol; or X is N, and P 1 is H or a hydroxyl protecting group, and X and P 2 combine to form optionally masked amino; or P 1 and P 2, together with the atoms to which each is attached, combine to form an aminal or 1,3-oxazolidin-2-one-5-yl; and wherein said intermediate in the synthesis of eribulin is a compound of formula or a salt thereof: R 1 is H or -CH 2 X 1 CH 2 CH=CH 2, wherein X 1 is O, -C(R 9) 2 -, or NP 5, and wherein each R 9 is independently H or -COOR 10, P 5 is an N -protecting group, and R 10 is C 1-6 alkyl; R 2 is H or a hydroxyl protecting group, R 3 is C 1-6 alkyl ether, and R 4 is H; R 2 is H or a hydroxyl protecting group, and R 3 and R 4 combine to form a double bond; or R 2 and R 3 combine to form a bond, and R 4 is H; R 5 is H or a hydroxyl protecting group, and R 6 and R 7 combine to form a double bond; or R 5 and R 6 combine to form a bond, and R 7 is H; R 8 is H, and P 4 is H or a hydroxyl protecting group; or R 8 and P 4 combine to form a double bond; R 2 is H or a hydroxyl protecting group, R 3 is C 1-6 alkyl ether, and R 4 is H; R 2 is H or a hydroxyl protecting group, and R 3 and R 4 combine to form a double bond; or R 2 and R 3 combine to form a bond, and R 4 is H; R 5 is H or a hydroxyl protecting group, and R 6 and R 7 combine to form a double bond; or R 5 and R 6 combine to form a bond, and R 7 is H; R 8 is H, and P 4 is H or a hydroxyl protecting group; or R 8 and P 4 combine to form a double bond; each P 3 is independently H or a hydroxyl protecting group; and X is O, and each of P 1 and P 2 is independently H or a hydroxyl protecting group, or P 1 and P 2, together with the atoms to which each is attached, combine to form a cyclic protected diol; or X is N, and P 1 is H or a hydroxyl protecting group, and X and P 2 combine to form optionally masked amino; or P 1 and P 2, together with the atoms to which each is attached, combine to form an aminal or 1,3-oxazolidin-2-one-5-yl; and X is O, and each of P 1 and P 2 is independently H or a hydroxyl protecting group, or P 1 and P 2, together with the atoms to which each is attached, combine to form a cyclic protected diol; or each of P 1 and P 2 is independently H or a hydroxyl protecting group, or P 1 and P 2, together with the atoms to which each is attached, combine to form a cyclic protected diol; X is N, and P 1 is H or a hydroxyl protecting group, and X and P 2 combine to form optionally masked amino; or P 1 and P 2, together with the atoms to which each is attached, combine to form an aminal or 1,3-oxazolidin-2-one-5-yl; and P 1 is H or a hydroxyl protecting group, and X and P 2 combine to form optionally masked amino; or P 1 and P 2, together with the atoms to which each is attached, combine to form an aminal or 1,3-oxazolidin-2-one-5-yl; and wherein said intermediate in the synthesis of eribulin is a compound of formula or a salt thereof: