New 1alpha,25-dihydroxy-19-norvitamin D3 analogs with a frozen A-ring conformation

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

Since the discovery of vitamin D a century ago, a great number of metabolites, analogs, hybrids and nonsteroidal VDR ligands have been developed. An enormous effort has been made to synthesize compounds which present beneficial properties while attaining lower calcium serum levels than calcitriol. This structural review covers VDR ligands published to date.

Strategies for the Synthesis of 19-nor-Vitamin D Analogs

1α,25-Dihydroxyvitamin D₃ [1α,25-(OH)₂-D₃], the hormonally active form of vitamin D₃, classically regulates bone formation, calcium, and phosphate homeostasis. In addition, this hormone also exerts non-classical effects in a wide variety of target tissues and cell types, such as inhibition of the proliferation and stimulation of the differentiation of normal and malignant cells. However, to produce these actions, supraphysiological doses are required resulting in calcemic effects that limit the use of this natural hormone. During the past 30 years, many structurally modified analogs of the 1α,25-(OH)₂-D₃ have been synthesized in order to find derivatives that can dissociate the beneficial antiproliferative effects from undesired calcemic effects. Among these candidates, 1α,25-(OH)₂-19-nor-D₃ analogs have shown promise as good derivatives since they show equal or better activity relative to the parent hormone but with reduced calcemic effects. In this review, we describe the synthetic strategies to obtain the 19-nor-D₃ derivatives and briefly describe their physiological activities.

A-Ring Synthons of 19-Nor Type Vitamin D Derivatives

Various 19-nor vitamin D analogs, which lack the methylene group at C19, exhibit significant vitamin D (VD)-related biological activities, but generally show a reduced calcemic side-effect compared with VD itself. Among them, paricalcitol is already used clinically for treatment and prevention of secondary hyperparathyroidism associated with chronic renal failure. Therefore, considerable synthetic efforts have been directed towards 19-nor VD analogs, focusing especially on A-ring synthons suitable for use in convergent synthetic strategies based on coupling of CD-ring synthons and A-ring synthons. For example, we have recently developed a new synthetic route to A-ring synthons from linear dienes based upon a ring-closing olefin metathesis strategy. Here, we review recent synthetic approaches to A-ring synthons for 19-nor VD derivatives.