Sunthetic Studies of Microsclerodermins. A Stereoselective Synthesis of a Core Building Block for (2 S,3 R, 4 S,5 S,6 S, 11 E)-3-Amino-6-methyl-12-(4-methoxyphenyl)-2,4,5-trihydroxydodec-11-enoic Acid (AMMTD)