only in the way that the different groups in the molecules are oriented in space stereoisomers have the same molecular weight chemical composition and constitution as another but with the atoms grouped differently that is certain identical chemical moieties are at different orientations in space and therefore when pure have the ability to rotate the plane of polarized light however some pure stereoisomers may have an optical rotation that is so slight that it is undetectable with present instrumentation all such isomeric forms of these compounds are expressly included in the present invention each stereogenic carbon may be of r or s configuration although the specific compounds exemplified in this application may be depicted in a particular configuration compounds having either the opposite stereochemistry at any given chiral center or mixtures thereof are also envisioned when chiral centers are found in the derivatives of this invention it is to be understood that this invention encompasses all possible stereoisomers the terms u201c optically pure compound u201d or u201c optically pure isomer u201d refers to a single stereoisomer of a chiral compound regardless of the configuration of the compound salinosporamide a exhibits a molecular structure having a variety of functional groups lactone alkylhalide amide hydroxide that can be chemically modified to produce synthetic derivatives accordingly exemplary invention compound salinosporamide a provides an excellent lead structure for the development of synthetic and semisynthetic derivatives indeed salinosporamide a can be derivatized to improve pharmacokinetic and pharmacodynamic properties which facilitate administration and increase utility.