Studies on the Structure-Toxicity Relationship of AK-Toxin, a Host-Specific Toxin to Japanese White Pear, Produced by Alternaria kikuchiana: Synthesis of Methyl (4S,5R)-4-(N-Acetylphenylalanyl)oxy-5,6-epoxy-5-methylhex-2(E)-enoate and Its Stereoisomers.