JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="left-panel">
        <div class="collapse-pane">OBSERVÇÃO</div>
    </div>
    <div class="chunk">Bloco de texto</div>
    <div class="chunk">Bloco de texto</div>
    <div class="chunk">Bloco de texto</div>
    <div class="clear"></div>
</div>

CSS

body {margin: 0;}

.container {
    position: relative;
}
.left-panel {
    background-color: #DDD;
    min-height: 110px;
    height: 100%;
    width: 40px;
    position: absolute;
    left: 0;
    top: 0;
    margin: 0 15px;
}

.collapse-pane {
    position: absolute;
    top: 50%;
    left: 50%;
    background: red;
    -moz-transform: translateX(-50%) translateY(-50%) rotate(-90deg);
    -webkit-transform: translateX(-50%) translateY(-50%) rotate(-90deg);
    transform:  translateX(-50%) translateY(-50%) rotate(-90deg);
}

.chunk {
    height: 50px;
    width: 85%;
    background-color: #f2f2f2;
    float: right;
    padding-left: 10px;
    margin-bottom: 10px;
}
.clear {clear:both;}